A History of Color: The Evolution of Theories of Lights and Color [1st ed.] 978-94-015-3941-8, 978-94-007-0870-9

Table of contents :
Front Matter....Pages i-2
Color theory in the ancient world....Pages 3-16
The Middle Ages....Pages 17-34
The Renaissance....Pages 35-49
Light, color and vision during the scientific revolution....Pages 50-76
Newton....Pages 77-87
From Newton to Young....Pages 88-111
Classical-romantic color theory in Germany....Pages 112-125
Disorders of color vision....Pages 126-132
The mixing of colors....Pages 133-140
The trichromatic theory....Pages 141-164
Hering’s four-color theory Zone theories ....Pages 165-174
Anatomy and physiology of the visual system between 1600 and 1900....Pages 175-190
The twentieth century....Pages 191-246
Back Matter....Pages 247-282

Citation preview

A HISTORY OF COLOR

ROBERT A. CRONE

A History of Color The Evolution ofTheories of Lights and Color

Reprinted from Documenta Ophthalmologia, Volume 96, No. 1-3 (1999)

Kluwer Academic Publishers DORDRECHT I BOSTON I LONDON

Library of Congress Cataloging-in-Publication Data

Crone, Robert A. (Robert Arnold) A history of color the evolution af thearies of light and color by Rabert A. Crane. p. cm. Adaptation of, Licht, kleur, ruimte I Rabert A. Crone. Inc 1udes index. 1. Color--Histary. I. Crane, Rabert A. (Rabert Arnold). Licht, kleur, ruimte. Ir. Title. aC494.7.C76 1999 535.6·09--dc21 98-51580

Published by Kluwer Academic Publishers, P.O. Box 17,3300 AA Dordrecht, The Netherlands Sold and distributed in North, Central and South America by Kluwer Academic Publishers, 101 Philip Drive, Norwell, MA 02061, U .S.A. In all other countries, sold and distributed by Kluwer Academic Publishers Group, P.O. Box 322, 3300 AH Dordrecht, The Netherlands

02-06-00-200 ts

ISBN 978-94-015-3941-8 ISBN 978-94-007-0870-9 (eBook) DOI 10.1007/978-94-007-0870-9 Softcover reprint of the hardcover 1st edition 1999

All Rights Reserved C 1999 Kluwer Academic Publishers Reprinted 2000 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner.

Contents Preface Color theory in the ancient world Empedocles' four elements and four colors The four-color doctrine Atomism and idealism: Democritus and Plato The empiricism of Aristotle The influence of Plato and Aristotle on science The Hellenistic and Roman era Neoplatonism The end of ancient scholarship

3 4 5 6 8 11 12 16 16

" The Middle Ages The early Middle Ages The visual science of the Islamic world The controversy about visual rays Ibn AI-Haytham (Alhazen) Alhazen's theory of vision Colors The refraction of light The science of vision and colors in the prime of the Middle Ages The perspectivists

17 17 19 20 22 22 25 26 27 30

'" The Renaissance Color in the Renaissance Optics in the Renaissance Johannes Kepler

35 36 39 44

IV Light, color and vision during the scientific revolution The scientific revolution Kepler and Galileo Bacon, Gassend and Descartes Descartes and vision New theories of light and color The speed of light The refraction of light The rainbow The chemical colors The color theories of opponents of the corpuscular hypothesis

50 50 50 51 54 58 64 65 68 71 73

V Newton A new theory of light and color Newton's color system The barycentric system The physiology of color vision VI From Newton to Young The reception of Newton's color theory Supporters of the medium hypothesis Intermezzo: achromatic lenses Supporters of the corpuscular hypothesis Conservative Aristotelians Practitioners on the classification of colors Three-color printing The first color triangles Butterflies and color-tops The start of color physiology The retina sensitive to three sorts of light? Thomas Young Theory of light Fresnel Invisible light Theory of color vision

77

78 83 85

86 88 90 91

92 93

94 98 99

100 102 103 104 106 107 108 108 110

VII Classical-romantic color theory in Germany Runge Goethe Intermezzo: subjective colors before Goethe Back to Goethe Schopenhauer

112 113

VIII Disorders of color vision Dalton Goethe Schopenhauer Seebeck

126 127 130

IX The mixing of colors Primary colors and the mixing of pigments Optic color mixing Wünsch

133 133 134 134

115 119

120 121

131

132

Chevreul Voigt, Young and Forbes Helmholtz Mixing spectral colors

135 135 137 138

X The trichromatic theory Helmholtz Grassmann Limitations of Grassmann's system Maxwell Colorimetry The fundamental sensation curves Trichromatism and dichromatism Arthur König Anomalous trichromatism Psychophysics Aubert and Mach: color as subjective quality

141 142 143 147 147 149 154 156 158 159 160 163

XI Hering's four-color theory Zone theories Theory of the four opponent colors Fick's hypothesis Zone theories

165 166 170 171

XII

Anatomy and physiology of the visual system between 1600 and 1900 Anatomy of the retina The neural structure of the retina Anatomy of the visual pathways The duplicity theory Day-blindness and night-blindness Visual pigment The Purkinje shift The photopic luminous efficiency function (V lambda function) Dark adaptation XIII The twentieth century The quantum theory The impact of the quantum theory on the science of vision and color The physics of color Photochemical processes

175 175 178 179 181 183 185 186 187 189 191 191 193 193 194

The quantum theory and the limits of vision The absolute threshold of vision The relative threshold of vision The relative thresholds of color vision Our spectral window and the quantum theory Other important aspects of the twentieth century color theory The further development öf color theories The trichromatic theory Luminance and color Zone theories Electronmicroscopy of the retina New facts about color vision defects Heredity Tritanopia Monochromatism The visual pigments The rod-pigment The cone-pigments Retinal densitometry Cone histochemistry Microspectrophotometry The structure of the cone pigments The evolution of color vision The neurophysiology of the retina Action potentials The horseshoe crab The visual nerve of the frog The receptive field Stimulation of the retinal ganglion cells with colored lights Electrophysiology of the cones Opponent processes The advantages of an opponent organization Color and luminance channels from retina to visual cortex Color psychology in the twentieth century The classification of colors The names of colors Contrast The influence of boundaries Color adaptation and color constancy The cortical color mechanism Functional specialization in the areas of the visual cortex

194 194 195 196 197 198 199 200 202 205 209 212 212 213 214 214 214 215 217 219 219 220 222 223 224 224 225 226 227 228 '229 232 232 233 233 234 236 237 239 241 243

Appendix and synopsis; what is color? Color and prescientific man The history of color theory Aristotle Alhazen, Bacon, Kepler Mechanicism and the subjectivity of the concept of color From Newton to the trichromatic theory Hering Modern color ,physiology The future of color science

247 247 248 248 248 249 249 250 251 251

Notes

253

ACknowledgments

261

References

261

Index

277

Documenta Ophthalmologica 96: 1-282, 1999. © 1999 Kluwer Academic Publishers.

Preface

The history of color theory can only be understood in the context of the history of all natural sciences. Because, for insight into the smallest component, a comprehensive view of the whole is necessary [1 J

Goethe, Farbenlehre This book gives a survey of color theories between 500 BC and 2000 AD. Naturally it cannot provide more than a broad outline. Goethe needed more than 500 pages for the historical section of his Farbenlehre (1810). The Dutch ophthalmologist Halbertsma compressed color history from 500 BC to 1950 AD into 270 pages (1949). In view of the numerous new discoveries in the field of color which have been made in the second half of this century, I have continued this history up to the year 2000, limiting mys elf in the last century to the most important discoveries and theoretical developments. Rather than an exhaustive encyclopedic treatment of the subject, I have chosen for a more readable text, meant for everyone who is interested in color, vision and the history of natural science. Specialized technical knowledge is not required. The History of Color is an adaptation of my book Licht-Kleur-Ruimte (LightColor-Space) which was published in 1992 in Dutch. In that monograph I included ophthalmo10gical subjects and a large chapter on spatia1 vision. I have not touched on the pictorial and aesthetic aspects of color. In addition to the fact that I did not fee I qualified to do this, new books [2] on the subject have been published recently, such as Colour and Culture by John Gage. I have also 1eft the emotional effects of colors out of consideration. Bearing in mind the quotation at the beginning of this preface, I have described the his tory of color theory in relation to theories of light and vision and, in a wider connection, in the context of the history of natural science. For this reason, the word 'color' does not appear on every page. Up to 1600 there is more happening in the field of visual theory than color theory. During the scientific revolution the nature of light is the focus of attention. After Isaac Newton, color becomes the main theme of this book; after Thomas Young, color vision. It is not until the second half of the twentieth century that the

2 connection is made between color vision and the physiology of the nervous system. The book which aroused my interest in ancient ideas about vision was Theories of Vision from Al-Kindi to Kepler by David Lindberg. The M echanization ofthe World Picture by EJ. Dijksterhuis was also an important source of inspiration. A broad path can be traced through the field of color theory which, not without diversions, leads from Greek science to modem color physiology. In every era sideroads were constructed, which sometimes connected to form a broad path, but still came to a dead end. No theory ever arose spontaneously: in all cases, I have tried to trace the path which led to a given hypothesis. I am indebted to several friends for their comments: Wim Delleman, Henk Spekreijse, Henk van der Tweel (t 1997), Pieter Stoutenbeek. My special thanks go to Hans Vos for his detailed (and often merciless) criticism. I also thank the translator Kathleen Boet-Herbert who always suggested the right English term for a clumsy Dutch word. The Dr. F.P. Fischer Stichting generously provided the funds for the translation. My publisher, Kluwer Academic Publishers, has helped me greatly with the editing of the manuscript.

I Color theory in the ancient world The Greeks were the first to start thinking as philosophers about vision and colors. They started inquiring into phenomena of which there was no previous knowledge. How could a doctrine of color vision be evolved, starting right from the beginning? Anyone who, with no backing, begins to think about vision, is immediately confronted by a dilemma. Do objects direct colors and shapes at us? Or does the eye investigate the world like an explorer? There are arguments for both hypotheses, but they both encounter formidable obstac1es. At first sight it seems obvious that vision is an active process. A person looks out of his eyes, he directs an inquiring gaze at objects and their color. If seeing is an activity, then something must emerge from the eye. That might be extremely fine matter, a sort of gas, as Pythagoras (c. 550 BC) thought. 'Visual rays', also called 'ocular rays', belong to the same concepLln view of the Greek enthusiasm for geometry, the idea of something emerging from the eye in a straight li ne (extramission) was attractive. The theory of visual rays was also based on popular tradition. It was assumed that the eyes contained 'fire' You only need to suffer a blow on the eye to see the ftames! It was generally believed that cats' eyes emitted light and could see in the dark. At the present time the alternative theory, that the visible world comes to us, the theory of 'intromission' , is thought to be the correct one. But the theory is far from self-evident. How can a mountain come to a thousand people at the same time? Do forms and colors detach themselves - thousandfold - from the mountain, so extremely reduced in size that they can enter our eyes? It sounds improbable. The idea of an optic image in the eye was not formulated until around 1000 AD by the Arabian scholar, Ibn al-Haytham, best known in the West as Alhazen; this idea had never occurred to the Greeks. For the Greeks, who were just starting on the difficult path of science, there was as much to say for a theory which took into consideration the intentionality of vision, the perception, as one based on the receptivity, the sensation. The ancient natural philosophers seI dom rejected an alternative completely; their ideas were usually a combination of both points of view. Some emphasized the active role of vision, others the passive role. For the modem scientist, the Greek scholars are exemplary in their attempt to concentrate on the permanent and essential factors behind the mass

4 of varying observations. This is still the aim of science: to reduce countless substances to a few elements, to classify innumerable plants and animals into a small nu mb er of phyla, to demonstrate what remains constant in spite of all changes: the total amount of matter or the total amount of energy. In modern science this reduction is the result of centuries of investigation. For the ancient Greek scholars, however, it was a starting-point. Once the essential had been formulated, the world of everyday experience could be disregarded as trivial. A typical representative of this train of thought is Parmenides of Elea (c. 500 BC). He demonstrates perfectly the Greek search for the permanent, the absolute. Distancing himself from everyday events, he states that the variety ofthings, their shapes and colors, is only a guise, no more than an appearance. Parmenides had a great deal of inftuence. The distinction between illusion and reality has remained an important theme in the study of visual perception. The theory of vision and colors was further elaborated by Empedocles and Democritus, two widely differing pre-Socratic natural philosophers. Empedocles' Jour elements and Jour calors

Empedocles of Akragas (490-435 Be; Akragas is the present-day Agrigento) is the first Greek philosopher to write on color. He is a many-sided genius: poet, philosopher, doctor and priest. Far from shutting himself up in the ivory tower of pure science, Empedocles travels through Sicily as a prophet and miracle-worker, surrounded by a host of followers. For Empedocles everything that is permanent is fourfold: fire, water, air and earth are the 'roots' of all things. These 'elements' are represented by the sun, the sea, the sky and the earth. The elements are ungenerated, indestructible, qualitatively unalterable and homogeneous throughout. Empedocles' idea is that, by the mixture of water, earth, air and sun there come into being the shapes and colours of all mortal things that are now in being, put together by Aphrodite [1]. There is continuous mixing and separation of elements, attracted to each other by love and repulsed by each other through hate. Empedocles believes in both extramission and intromission. He is convinced that the eyes shoot fire. The eye can be compared to alantern: the ocular fire takes the place of the buming oil and the transparent pupil functions as the window of transparent horn. On account of this comparison Empedocles can be regarded as an extramissionist. But on the other hand he also declares that objects produce an emanation. There are pores in the eye, of exactly the right size and shape, which not only allow the ocular fire to leave the eye but also admit the emanations arising from outside objects. These include the colors:

5 Thus black and white and every other colour will appear to us as produced by the encounter of our eyes with something which moves in the direction of the eyes. And that any particular colour which we see is neither the object which comes towards the eye nor the eye which is met, but rather something which is produced between them [2]. Analogous to the four elements: air, water, fire and earth - there are four basic colors: white, black, red and yellowish green - and four sorts of pores through which they enter the eye. The pores see to it that things which are alike in man and in the outside world come into contact with each other. As Empedocles says, With earth we see earth, with water water, with air the heavens, but with fire destructive fire [I]. This principle applies to all the senses, not only to the eye, which resembles light because of its transparency. It also applies to knowledge of the divine. Because the divine is in us, we may know the gods. This idea, which was also later expressed by Plotinus, inspired Goethe to a poem which can be found in his Farbenlehre: Were the eyes not sun-like, How could we see the sun? Lived not in us the power of God, How could we delight in the Divine? [3]. The Jour-color doctrine After Empedocles had formulated his doctrine of the four elements, Greek thought became obsessed with the number four. Empedocles distinguished water, air, fire and earth - and the colors black, white, red and yellowish green. Aristotle added a quartet of qualities: warm, dry, damp and cold. Hippocrates produced four body fluids: black bile, blood, yellow bile and phlegm. These fluids were responsible for the four 'humors': melancholic, sanguine, choleric and phlegmatic. Together with the four seasons, the four ages of man (child, youth, man and greybeard) and the four parts of the day, a total was reached of eight tetrads. The tetrads formed one of the main pillars of the school of Galen (129-179), which influenced medical- and not only medical - thought up to weIl into the 17th century. Empedocles' four basic colors surprise us. In the first place we might ask ourselves whether white and black are colors at all. Are they not words used to express the absence of color? And in the second place: where is blue, which would appear to be a noticeable, primary, unmixed color? Was the color vision of the ancient Greeks underdeveloped? This conclusion was reached by Gladstone (1858), prime minister of England and an amateur philologist. He was struck by the paucity of color words in Homeric Greek. Others have

6

also th0Ught that human color vision developed in historic times, red being the first color to be recognized [4]. But the 'color-darwinists' were wrong; the ancient Greeks were not colorblind but only lacked (abstract) terms for color [5]. Recently the color vocabularies of a hundred languages have been investigated [6]. In 17 languages there were only words for Empedoc1es' fOUf colors. This does not mean that the people belonging to the 17 language groups see few colors. Empedoc1es' choice of four colors was apparently not arbitrary, but reflected the developmental level of the Greek language at that time. There was a world of colors to be seen before the language had words for them. This fact would appear to refute the first verse of St. John's Gospel: 'In the beginning was the Word' (or, as Wittgenstein [7] puts it: 'The limits of my language are the limits of my world'). Atomism and' idealism: Democritus and Plato

Democritus of Abdera (460-370 BC) was in many respects the converse of the adventurous Empedoc1es. Democritus traveled a great deal too, to Egypt and Babyion, but his only aim was to increase his knowledge. He was an encyc10pedic scholar who practiced mathematics, astronomy, medicine and music. He also wrote on the art of painting and on perspective. Practically nothing remains of his written work but his ideas have been preserved thanks to his follower Epicurus (341-270 BC) and the Roman poet-epicurean Titus Lucretius Carus (95-52 BC), whose didactic poem De rerum natura is the best exposition of Democritus' work which we have. Democritus is an exponent of the theory that the visual world comes to us, although extramission is not entirely absent; seeing is achieved by the 'emphasis', the meeting in front of the eye between the image of the outside world and the reflection from the cornea. The images of the outside world, the eidola, are a sort of minimalized flying outer skins of objects. Lucretius calls them simulacra and compares them with the discarded skin of a snake. In W.E. Lenard's translation: And thus I say thateffigies of things, And tenuous shapes from off the things are sent, From off the utmost outside of the things, Which are like films or may be named a rind ... As when the locusts in the summertime Put off their glassy tunics, or when calves At birth drop membranes from their bodies' surface, Or when, again, the slippery serpent doffs Its vestment amongst the thorns [8]. The philosophy of Democritus (and of his mentor Leukippos) is based on the atomistic theory. The immutable being, for Parmenides one, for Empedoc1es

7 fourfold,is multiplied by Democritus to become infinite. Nature consists of innumerable 'seeds', indivisible atoms, unchanging and distributed through empty space. The atoms are in perpetual motion, at the mercy of blind chance. The whole natural world, inc1uding the soul, is formed of conglomerations of these atoms. Democritus adopted Empedoc1es' four colors - black, white, red and yellowish green - but, being an atomist, he did not connect these with the four elements. The color atoms have different shapes: the white atoms are round and smooth, the black atoms, which throw shadows, are rough and irregular. The red are round, like the atoms of fire, but bigger. In addition to primary colors, Democritus also described compound colors: green, brown and the blue of woad. He certainly used this knowledge in his book on painting. The atomistic theory requires that colors are really only colorless atoms, 'seeds'. Colors only have a subjective reality. Seeds receive no property of color, and yet Be still endowed with variable forms From which all kinds of colors they beget [9]. The color atoms only become color if they bring the soul atoms into motion. The soul atoms are extremely small. First I aver, 'tis superfine, compound Of tiniest partic1es. That such the fact Thou canst perceive, if thou attend, from this: Nothing is seen to happen with such speed As what the mind proposes and begins ... But what's so agile must of seeds consist Most round, most tiny, that they may be moved When hit by impulses slight [10]. Colors and other sensory qualities are thus not present in the objects themselves. Sensory knowledge is not knowledge of the objects themselves and is in fact 'second quality' knowledge. Democritus is here the first to formulate a problem with which every logically thinking atomist will be confronted: sensory qualities can only be considered as 'secondary qualities'. I shall return to this dilemma when the history of color is two thousand years further on. The materialistic atomistic theory of Democritus was not accepted by the Christians, but was never completely forgotten and took a new lease of life at the time of the Scientific Revolution. Although Democritus with his atom theory was far removed from Parmenides with his 'one', he returns to Parmenides' ideas with his deprecation of the sensory. For both of them the world of the senses is only an illusion, a metaphor.

8

The same view was held by the philosopher who was to completely eclipse his contemporary Democritus: Plato, the founder of the Academy (387 BC). Plato (428-347) carried the distinction between appearance and essence to extremes. He assumed that we can grasp a supersensory, immaterial world of forms and ideas. Plato found support for his concept of a supersensory second world of everlasting truths in mathematics. You can draw triangles in the sand to help your imagination, but the geometrical principles which are revealed are as permanent as the drawings are transitory. Plato had little interest in the natural sciences. His only concept of the natural world is to be found in one of his last works the Timaeus. It is more a fantasy than a doctrine based on empirical facts. A mathematically minded Divine Craftsman, the Demiurg, makes from a world of chaos an ordered system. This is the cosmos, invested with a universal soul. The world is built of elements resembling Democritus' atoms, but they are pure mathematical entities: regular polygons. Plato also considered the colors to be corpuscular. In the Timaeus Plato adopted Empedocles' basic colors: white, black, red and yellowish green; related to the four elements, which themselves were built up of regular polygons. Plato mentioned compound colors but gave no explanation of them. The colors are the object of the visual process and light is the medium. For Plato light has a metaphysical status. He calls the sun 'the child of Good' and considers the eye, which can see the light, as the organ most closely allied to the sun. Vision is for Plato the result of a tripIe process. The eye emits fire; this fire combines with daylight to form one beam (the synaugeia). Colors stream out of the object and are added to the beam. To quote from the Timaeus: color, a flame which streams off from bodies of every sort and has its particles so proportioned to the visual ray as to yield sensation [11]. In this way not only the two aspects of vision - active and passive - are given their due, but the function of light is also included. This concept was adopted by Plato's great pupil Aristotle. The empiricism of Aristotle For Greek science Democritus' and Plato's abstractions had little to offer. The ideas of Aristotle, who approached reality in a more concrete and empirical manner, were more fruitful for ancient science. Aristotle of Stagyra (384-322 BC) was the founder of the Lyceum (335 BC). Instruction was often given while the participants were strolling between the pillars. For this reason his pupils were nicknamed the peripatetics. The color theories of the previously mentioned philosophers were fragmentary, but Aristotle had comprehensive ideas on color. There is even aseparate book, On calors that is thought to

9

have been written by his pupil and assistant Theophrastus [12]. He therefore deserves more attention in this context than the others. There is a fundamental difference between Aristotle and his mentor: Aristotle is very interested in empirical science. He has an encyclopedic mind which he directs towards many aspects of science. His studies of marine zoology, for instance, are famous. Aristotle adopts Empedocles' four elements. He combines them with four primary qualities, taken from the realm of touch: cold, warm, dry and wet. The earth is cold and dry, water cold and wet, air is warm and wet and fire is warm and dry. Mixing the four primary qualities gives rise to the secondary qualities such as scent and color, in this process subdivision usually occurs into 'extreme qualities' and 'intermediate qualities'. Aristotle rejects Plato's transcendental world of ideas. The universal does not belong to aseparate world but is incorporated in the world of particular experience. He also rejects Democritus' system: he has no use for the atoms themselves or for the spaces between the atoms. While Democritus reduces everything to quantitative entities, it is qualities which have fundamental reality for Aristotle. They are the 'forms' into which 'matter' is poured. Finally Aristotle rejects Parmenides' idea that all change is an illusion. He does this by introducing terms like 'potential', 'actualization' and 'development'. An oak is the actualization of the potential hidden in an acorn. According to Aristotle, development is an inherent quality of life. The developmental process is purposeful; in addition to causal processes (energeia) there are purposeful processes (entelecheia). Aristotle's color theory [13] forms part of a general theory of perception. Perception is the process by means of which forms from the outside world (without the accompanying matter) affect the sensory organ, just as the wax in a signet-ring receives the impression of the seal without removing any of the iron or gold of which the ring is made. The impression actualizes the potential present in the sensory organ. The paradox that perception is passive is nullified by this formula: the impression received by the sensory organ represents the passive side of the process, the mobilization of the potential in the sensory organ itself is the active side of perception. According to Aristotle, every sensory organ is sensitive to specific qualities. In the case of the eye these are the colors. In order to achieve definitive perception other, less specific, information is often necessary: shape, movement, similarity to other objects. The final identification, in which associative processes also playapart, does not take place in the sensory organ but in the sensus communis, the common organ of sense. Because Aristotle considers that the soul is situated in the heart, he also places the sensus communis in

10

that organ. (The brain, which was thought by Alkmaion of Croton, c. 400 BC, to house the thought process, was designated by Aristotle to cool the blood.) Application of Aristotle's general sensory theory to the sense of sight signifies that the active projection of visual rays by the eye is rejected. Aristotle breaks with the extramission theory [14]. Nor is there a place for material intromission in Aristotle's system. It is the shape of visible objects which affects the eye, not the material. Like Plato, Aristotle postulates a conducting medium, the air. But the stars are seen through a different medium, the ether. This is the fifth, divine element, quinta essentia. The medium in the eye itself is the eye fluid. In this transparent medium light actualizes the potential of colors to affect the eye. A distinctive feature of this train of thought is that actualization is being considered and not movement in space. The actualization occurs twice: in the first place the transparent medium is actualized - that is light. Light is not a substance but an accidens, it is astate of something else. The real object of vision is color, which is a property of the surface of things. Color produces a second actualization in the medium, which is perceived by the subject. Colors therefore do not move towards us through a medium, but affect us by causing a change in the medium. Perception is an instant process; light, colors and shapes do not need time [15]. For us, the receptive role of the eye is self-evident. But at the time the theory failed to convince; even Aristotle's pupil Theophrastus was skeptical. The most important theorists of subsequent centuries (Euclid, Ptolemy, Galen, for example) preferred a radical extramission theory. It was not until a thousand years later that the visual rays were emphatically rejected by Ibn al-Haytham (Alhazen). Concerning the nature of colors themselves, Aristotle bases his ideas on the colors white and black, which are directly associated with light and darkness and form the 'extreme qualities'. The other colors are mixtures of light and darkness, 'intermediary qualities' . Aristotle observes that there are great differences in the brightness of objects, whereas no colors are brighter than white or darker than black. He calls green the happy medium between light and dark; purpie is one of the dark colors. Just as (according to Aristotle) there are seven tastes, and seven tones in the musical octave, there are also seven colors: white, yellow, red, green, blue, purple and black [16]. Aristotle writes extensivelyon the process of mixing which produces the colors. It is not simply juxtaposition of light and dark: that would only produce grey. The mixing is more like a melting-down process, a chemical reaction. In the peripatetic book 'About colors' a nice illustration of this process is given: a mash of purple snails is grey at first, it only becomes purple after it has been boiled for some time.

11

Light and dark can also be mixed in another way: when the diaphanous medium is semi-transparent. When we look at light through a semitransparent medium it becomes yellow or red, as in the case of the setting sun. On the other hand, if we look at darkness through this medium it becomes blue. Thus, according to Aristotle, the colors help us to estimate distance. The 'apparent' colors of the rainbow form aseparate group, distinct from the normal 'true' colors of the objects themselves. They are not attached to an object and alter their position when the observer moves. The colors of the rainbow are red, green and purple. These apparent colors, which are brighter than white, artists cannot make by mixing pigments. Red, which is on the opposite side of the rainbow to purple, is dosest to the light. The appearance of yellow in the rainbow is due to contrast: the red is whitened by its proximity to green. In other contexts (see above) Aristotle points out the relationship between yellow and white. There is thus some ambiguity in Aristotle's linear color dassification: on the one hand he is indined to place yellow between red and green because that is the arrangement in the rainbow, on the other hand he considers yellow to be the color which is dosest to white. This dilemma is still to be found in the writings of the last two Aristotelians of color theory: Goethe places yellow between red and green, Schopenhauer between red and white. Aristotle describes the rainbow as a reftection of the sun in the myriad of reftecting surfaces of a doud. He discards the ancient Greek theory according to which the circular form of the rainbow is due to its reftection from a concave doud. Instead, he gives an astronomical-geometrical explanation according to which the reftection is from a hypothetical heavenly vault [17]. Aristotle asks hirnself (in De anima) whether a numerical system forms the basic principle of the colors, as is the case with musical tones: We may regard all of these colors as analogous to the sounds that enter into music, and suppose that those involving simple ratios, like the concords of music, may be those generally regarded as most agreeable; as, for example, purple, crimson, and some few such colors, their fewness being due to the same causes which render the concords few [18]. The influence of Plato and Aristotle on science

Aristotle, who chose a midway course between the materialism of Democritus and the idealism of Plato, had an enormous inftuence on science, even up to the present day. Democritus' teaching was rejected by the Christians in the Middle Ages and only became an important riyal at the time of the scientific revolution. Plato's teaching is another matter: In the subsequent two thousand years Platonism has repeatedly competed with the Peripatetic school for the supremacy. Hellenistic Neo-Platonism had great inftuence in

12 the early Middle Ages and in the early period of Arab scholarship. At the zenith of Arab science Aristotle was predominant, and this school gave an important impulse to the scholastic renaissance of science. The early-modem science of Kepler, Galileo, Huygens and Newton demonstrates Platonic (and Democritan) inftuences: emphasis falls on the abstract, mathematical treatment of problems. In that quantitative atmosphere physics and cosmology ftourished. For biology and psychology, on the other hand, Aristotelian conceptual systems of forms and qualities are indispensable. Biologists think in terms of morphology, development and function. Psychologists cannot do without the idea of quality. Thus in the study of vision, color is a sensory quality which cannot be disregarded. It is not surprising that, even today, Aristotle's voice can still be heard.

The Hellenistic and Roman era Aristotle, mentor of Alexander the Great, stood at the threshold of a new era. Thanks to Alexander's conquests, Greek civilisation reached to the Indus and to Egypt. Aristotle was an important philosopher and embraced the whole of science in a comprehensive scope. After hirn philosophy and science took diverging paths. Scientists became indifferent to philosophy and philosophers restricted their activities more and more to ethical and religious matters. Athens remained the center of philosophy. Epicurus had his 'garden' there and Zeno his 'arcade' (Stoa). Epicurus (341-270 BC) was the prophet of the refined enjoyment of life; he supported the philosophy of Democritus. Thanks to Epicurus and his pupil Lucretius p. 6), the old atomistic theory has been preserved. Zeno (336-270 BC) propagates the ethics of responsibility. In his theories he is closer to Aristotle, and he is a vitalist. He considers that matter is continuous, but interpenetrated by an active principle, the pneuma (breath). The divine pneuma (logos spermatikos) shapes the world. Man also has pneuma, the breath of life, as directive power (hegemonikon). There is thus an analogy between man and the world, between mikrokosmos and makrokosmos. Alexandria became the scientijic center of the Hellenistic world. King Ptolemy, Alexander's successor in Egypt, founded there an academy of science, the Museum, with a large library. Numerous scholars worked in Alexandria, among whom the geographer Eratosthenes, who made precise measurements of the curvature of the earth, and Aristarchus, who argued that the earth revolved round the sun and on its own axis. Although no important new insights into color arose in the early Hellenistic period, two scholars should be mentioned who have contributed much to the theory of vision.

13

Euclid (c. 300 BC), the great mathematieian, was also interested in vision. He formulated geometrical explanations of why a tree in the distance looks smaller than one close by, and why a circle lying in the same plane as the eye appears as a line. Euclid thought in terms of visual rays diverging in straight lines from the eye. The figure enclosed by the visual rays is a cone whieh has its apex in the eye and its base at the level of the object looked at. Euclid's thesis that rays exist and that they progress in straight lines is the basis of all geometrical opties. Herophilus (c. 300 BC) was King Ptolemy Soter's personal physician and a great anatomist. He described the brain and the internaiorgans in detail. He also described all the components of the eye as far as they are visible without a microscope. Strangely enough, he described the optic nerve as a hollow tube. He considered that the transparent parts of the eye were the most important parts because, by their nature, they were so closely related to light. That strange transparent organ, the lens, had to be the seat of vision. Hanging as it was in a very thin membrane (aranea) like a spider in its web, it appeared to be the center of the organ of sight. Up to the seventeenth century many scientists did not doubt the fundamental role of the lens in vision. In the late Hellenistic period two other scholars made important contributions to the theory of vision, the astronomer Ptolemy and the celebrated physieian Galen. Claudius Ptolemy (c. 150 AD), who lived 450 years after Euclid and Herophilus, was one of the great scholars of the Hellenistic period. He was especially famous for his astronomie work (the Almagest), but also wrote a book on optics (Optica), which has unfortunately not been completely preserved. It is precisely the first part, which deals with color, light and the visual rays, which has been lost. Ptolemy continued the geometrie al optic work of Euclid. He made important discoveries in the field of binocular vision and he was not far from the correct explanation of stereoscopic vision [19]. He not only studied the laws of reftection (katoptriea) but also the laws of refraction (dioptriea). He determined the refractive angle at various angles of incidence on the passage of light from air to water and to glass. Although he was so intensely occupied with refraction, his explanation of the rainbow, in which he differentiated seven colors, was no more advanced that that of Aristotle. Ptolemy believed in both extramission and intrornission; visual rays exist which are of the same nature as light and color. On this point he was in agreement with his contemporary, Galen, p. 14. His ideas on perception were Aristotelian: color is what is primarily seen. Color is for sight what sound is for hearing. Subsequently, to achieve perception, secondary data are necessary, such as shape, position and movement, whieh are not specific for sight. These are the spatial characteristics of the visual impression which arise

14 in the sensus communis. The last phase of the visual process begins when the primary and secondary features of the visual object are subjected to judgment; only then does the visual sensation become perception [20]. Ptolemy was the first to describe how colors can be mixed, not only on the artist's palette but also 'optically' in the eye. Ptolemy painted colors on a wheel - probably a potter's wheel - which he rotated rapidly. In this way he obtained an impression of the time needed to make a single observation, and he could mix colors because the eye did not have enough time to distinguish the individual colors on the rotating wheel. (In the middle of the nineteenth century this method contributed a good deal to our knowledge of color vision.) It was also possible to mix individual colors 'optically' in another way: by looking at them at a distance. Thus a mosaic of brightly colored elements may even make a grey impression at a distance. As Ptolemy writes: Now we see how, because of distance or the speed of movement, the sight in each of these cases is not strong enough to perceive and interpret the parts individually' [21]. Galen (130-200 AD) represents the acme of Greek medical thought [22]. He was born in Pergamum and later became the personal physician of emperor Marcus Aurelius. At the same time he wrote an enormous number of medical and philosophical books. Galen's medicine was based on the theory of the four elements, qualities, body fluids and temperaments (p. 9). Galen also described four complexions to go with the four body fluids and temperaments: pale for phlegm, yellow for yellow bile, red for blood and dark for black bile. The pneuma theory of the Stoa occupies an important place in his physiology. The pneuma of life reaches the brain by means of the arteries and is transformed there into the finer pneuma of the soul. Galen situates the hegemonikon, the co-ordinating center of the physical and spiritual individual, in the brain. He regards the heart as a pump which regulates the 'ebb and flow' in the bloodvessels. Like Aristotle, Galen thinks teleologically. He is interested in the purpose of events, not in their cause. Galen made an intensive study of the anatomy of the eye, elaborating on Herophilus' work. He also recognized the chiasma, the X-shaped tract of the optic nerves before they enter the base of the brain. There the optic nerves reach the 'thalami' the word used for the cavities in the brain. In his theory of vision Galen reveals hirnself as an eclectic who makes use of both Stoic and Platonic elements. Color does not interest hirn. An important role is played by visual pneuma, which is part of the pneuma of the soul. Visual pneuma from the brain reaches the eye via the hollow optic nerve and leaves it subsequently through the pupil. Together with light it brings the air between the visual object and the eye into astate of tension, which

15

causes the air to become an extension of the visual pneuma. In this way Galen avoids the improbable intromission (the mountain does not have to come to the eye) and the equally improbable extramission (the pneuma does not have to reach the mountain). In much the same way as Plato's synaugeia, air plays an intermediary role between the visual pneuma and the color of the visual object. As soon as vision has been effectuated in the lens, the information about shapes and colors must naturally be reported to the cavities of the brain, the hegemonikon. This registers the changes in the lens and sends the pneuma back via the optic nerve. For a long time Galen's work had great authority. In the fourth century he was already considered to be the equal of Plato and Aristotle (Fig. 1.1) and for the Arabs he was as much a legend as Hippocrates. The triumphal march of Galen in the West began around the eleventh century. Because of Galen's unassailable authority, the ancient concept that the lens was the seat of vision was not doubted for fourteen centuries. This had an inhibiting effect

Figure 1.1. Galen playing a quartet with Plato, Aristotle and Hippocrates (Champier:

Symphonia Platonis, 1516).

16 on the development of ophthalmology and the theory of vision. It was the year 1604 before Kepler demonstrated that not the lens but the retina was the seat of vision. But at the end of the eighteenth century the famous anatomist Soemmering still believed Galen, that the soul was situated in the cavities of the brain.

Neoplatonism The neoplatonism of Plotinus (204-269) is a philosophical school of the latehellenistic period, inspired by Plato, but also by oriental mysticism. It has something to say about light and color and thus merits attention here. As mentioned above (p. 8), Plato called light 'The child of Good', but it is Plotinus who really created 'light metaphysics'. According to Plotinus everything originates from the One and its Emanation. Shining light naturally becomes the symbol for the highest metaphysical principle. Color arises when light mixes with matter. Color, therefore, has a lower status than light. Neoplatonism had considerable inftuence, not only in the Middle Ages, but also in the Renaissance. For a long time light metaphysics stood in the way of the correct understanding of color - as an essential feature of light itself. According to Boethius (480-524), the neoplatonic author of the famous De consolatione philosophiae, color was nothing more than a material incident, an accidens.

The end 01 ancient scholarship All the scholars mentioned in this chapter were Greeks. The contribution of the Romans to ancient scholarship is not large. As Horace remarked: 'When Rome captured Greece, Greek intellect and art captured Rome' (Graecia capta lerum victorem cepit). The most important contributions of Roman scholars lie in the field of civics and law, and in the fact that they popularized Greek thought. Lucretius (95-52 BC) has already been mentioned in this connection. Another popularizer of natural science was Cajus Plinius the EIder, the writer of the Naturalis Historia. He died AD 79 as a victim of the eruption of the Vesuvius. Long before the West-Roman Empire had been overrun by barbarians from the north, scholarship began to decline through poverty, immigration and the loss of contact with Greece. Philosophy had shrunk to lessons in practical living and the study of nature became the victim of esoteric and religious fantasies. The human spirit, disappointed in science and philosophy, started to look for a new domicile. Many found this in Christianity.

11 The Middle Ages

The early Middle Ages At first Christianity did not have a stimulating infiuence on scholarship. On the contrary, its attitude was frequently hostile. The new religion laid claim to all aspects of life and designated the study of nature as vanity. As the theologian Tertullianus (c. 160-220) expressed it: We have no need of an enquiring mind, after Jesus Christ, nor of investigations, after the Gospels [1]. Nevertheless, it is Christianity which, in the dark ages of Western Europe, preserved ancient scholarship and philosophy for posterity. A number of classic works (naturally, mainly those that were acceptable to the church) were preserved in the West, often in isolated monasteries. The fact that the church was able to do this was, to a considerable extent, due to Aurelius Augustinus (354-430), the creator of the Civitas Dei. Augustine, who later became bi shop ofHippo, was in his youth fascinated by (neo)platonism. After his conversion he continued to be an admirer of Plato and discerned a similarity between Plato's Demiurg and the Creator of the Jewish-Christian tradition. He considered that Greek philosophy was useful as long as it was a subordinate 'handmaiden' of theology. In the chaos of the migrant hordes, the remnants of ancient culture were best preserved in Irish monasteries. These had good contacts with the church in Spain. Later the Hibernian scholarship spread to England and, in the prosperous times under Charlemagne, to the land of Franks and Germans. In the cloister schools study and teaching mainly related to the Bible and the works of Augustine and other church fathers. The shadow of Plato and Aristotle hangs over the early Middle Ages. Because the knowledge of their work is incomplete their infiuence is paradoxical. As far as Aristotle, the great naturalist, is concerned, he is known for his Logic; on this is based the formalistic dogmatism often considered to be the typical element of scholasticism. All that is known of Plato, the idealist who was so indifferent to facts, is his biological and cosmological fantasy, the Timaeus. On this basis Plato becomes the patron saint of the liberal arts, the artes liberales.

18

lsidore, archbishop of Seville (c. 560-636) is one of those who helped to preserve the ancient scholarship (as handed down by the Roman encyclopedists like Varro and Plinius). He writes an encyclopedia of sciences and arts, the Etymologiarum sive Originum libri Xx. Here follows a specimen of his etymological way of thinking: Lac (milk) derives from its color, because it is a white liquor, for the Greeks call white leukos and its nature is changed from the blood: for after the birth whatever blood has not yet been spent in the nourishing of the womb flows by a natural passage to the breasts, and whitening by their virtue, receives the quality of milk [2].

Isidore writes on the color changes of the cameleon and the color of precious stones. He pays much attention to the color of apocalyptic horses and states that a hexagonal crystal has been found in the Red Sea which radiates all the colors of the rainbow when light falls on it. He sees the four elementary colors in the rainbow, but also endows these colors with mystic significance. Thus blue stands for heaven, purple for martyrdom, red for mercy and white for chastity. Isidore makes a sharp distinction between Lux, the radiating Substance, and Lumen, its effluvium. He is interested in the physics of the ancient atomists and acquainted the Middle Ages with the physical ideas of Epicurus, in spite of the fact that he deplored his moral views. The Venerable Bede (674-735), an early English scholar, is also interested in the liberal arts and writes De natura rerum, an encyclopedia in which he expands on Isidore's work and on the scholarship that had been preserved in the Irish monasteries [3]. He connects the four elements with the four main colors (which are not the same as those of Empedocles): the sun has the red color of fire, the air is blue, water purpie and the earth green. The rainbow, which contains the four main colors, arises when the rays of the sun shine on a concave cloud and return in the opposite direction, just as the sun shines on a vase of water and the brightness is sent back to the ceiling. Bede expands on the color of precious stones in his commentary on the Bible book Revelation, in particular the colors of the precious stones of which the Heavenly Jerusalem is built. For centuries to follow Western scholarship does not advance further than the receptive study of earlier material. Independent scientific thought was beyond the reach of the scholars; they drank from the antique springs and at the same time submitted to the authority of the church. They were not yet capable of formulating a theory of vision and color. This condition only changed centuries later, more specifically when the Western world became acquainted with Arab culture. For this reason an intermezzo follows in which the amazing development of visual science in the Islam is considered.

19

The visual science of the Islamic world

In the East-Roman empire, where the political stability was greater than in the Western empire, natural science did not prosper either. Over aperiod of a thousand years Byzantium contributed nothing to the science of vision. Medicine and physiology remained in the hippocratic-galenic tradition. But antique medicine and the ancient theory of vision were still a source of inspiration, although it was not the Byzantines who were inspired but the people of the Middle East, especially the Persians and the Arabs. The journey of Greek learning to the East [4] beg an with the conquests of Alexander the Great. Later Romans were captured by Persian kings and forced to live in Persia. Christians were forced over the Persian border by the Roman persecution. Heretics (Nestorians and Monophysites) fted to the East from the Byzantine orthodoxy. The Sassanids, kings of Persia, invited Greek philosophers to their country when the Aristotelian school in Edessa ceased to exist in 489 and the Neoplatonic school in Athens was closed by lustinian in 529. Now we are less than a century away from the moment that an Arabian prophet moved from Mekka to Medina, at which point a new era began in the Middle East. Mohammed, the prophet, brought with hirn the Koran, the original word of God which was revealed to hirn by the angel Gabriel. The new religion, the Islam, inspired the Arabs to the creation of a world power and a cultural identity. The conquering Arabs were tolerant in many respects. Non-Arabs were at first not forced to conversion. The Islam had a positive attitude towards learning. The prophet Mohammed wrote: 'The ink of scholars is more valuable than the blood of martyrs'. The golden age of Persian-Arabian culture was in the time of Harun al-Rashid (766-809), the legendary caliph from the Arabian Nights. He encouraged scholarship and sent out agents to buy old manuscripts. Syrian, Indian and Greek texts were translated into Arabic. Nestorian-Christian doctors acquired great inftuence at court; medicine was in high esteem in the Middle East. Many scholars were doctor, philosopher and priest at the same time. The science of vision also received new attention: again there were supporters and opponents of the theory of visual rays (AI-Kindi and Ibn-Sina respectively). Galenic ophthalmology was carried on by Hunayn Ibn lshaq al-Abadi. The optics and perception theory of Ptolemy were crowned by the work of Ibn al-Haytham (Alhazen) and Kamal al-Din al-Farisi.

20 The controversy about visual rays AI-Kindi (c. 850) was the first Arab philosopher. At the court of caliph AIMamum he was doctor, musician, mathematician and astrologer. He translated some of Aristotle's works into Arabic. He regarded the Koran as a creed for illiterates; Aristotle's creed was higher wisdom, only intended for enlightened spirits. In his theory of vision al-Kindi is unfaithful to his revered master. In his work on optics, which has become well-known in the Latin translation under the name De aspectibus he defends the extramission theory. AI-Kindi adopts a neo-platonic philosophy of nature in which the emanation of power is a fundamental principle. For hirn, vision is the emanation of visual power. In his theory of vision he therefore allies hirns elf with the optic tradition which runs from Empedocles via Euclid to Galen. In his opinion Democritus' theory of 'eidola' cannot be true: the Euclidian geometry of the visual rays makes it clear how a circular object can be seen in a certain direction as a line; if an eidolon had been given off by such an object we would have to continue to see it as a circle. AI-Kindi speculates on the blue color of the sky, which he attributes to a mixture of the darkness of the sky and the light of dust and water particles which are made luminous by the sun. Later Arab scholars - who are usually doctor and philosopher together adhere more closely to Aristotle's theory and reject the extramission theory. Ibn Sina (Avicenna, C. 1000) is still considered in the Arab world to be the 'prince of doctors'. He was personal physician to various caliphs and wrote the Canon of medicine. This great work was for centuries also popular in the West. As philosopher he was a consequent Aristotelian. The impossibility of visual rays is apparent from the following argument: through clear water we can see the bottom of avessei, nonetheless water is a compact non-porous material. How could a bundle of visual rays penetrate to the bottom of the vessel without the surface of the water rising? Galen's theory is also unacceptable: the air does not reach to the stars, so it can never be a participating elongation of the visual pneuma. Nor can Galen explain how we can see when the wind is blowing! Avicenna deliberates at length over the colors of the rainbow and comes to the final conclusion that the origin of these colors lies in the eye. Avicenna is the first to describe aseries of scales between white and black, for every tint and even for the achromatic grey [5]. It is a risky attempt because, according to Aristotle, colors are themselves the result of the mixture of light and darkness. But Aristotle had already recognized the grey scale which occurred when light and darkness did not form a compound but only underwent physical mixing. In addition to the Aristotelian linear arrangement of the various colors, Avicenna made another linear arrangement for each

21

separate tint from white through fuH color to black. It is the first attempt to make a two-dimensional classification of colors, which many centuries later was to be perfected by Newton. Hunayn Ibn lshaq al-Abadi (980-1037), a Christian who was caHed 10hannitius in the West, has been called the Erasmus of the Arab renaissance. He and his staff translated many of Galen's and Hippocrates' works. He also wrote medical books of his own; the most important is Ten treatises on the eye. Hunayn's book makes it clear that the Arabs adopted Galen's medical ideas at an early date. Hunayn's work was translated into Latin in the eleventh century, long be fore the West became acquainted with Galen through direct translations of the classical master's work. Hunayn's book includes the oldest known illustration of the anatomy of the eye (Fig. 2.1). It is astrange picture, partly section and partly frontal view. Following the example of Herophilus, the lens is placed in the middle of the eye and the optic nerve is hollow. Hunayn's visual theory is like Galen's. Under the influence of visual pneuma and light the air undergoes a transformation which makes perception possible. Colors also cause a transrnutation of the air.

OPTIC

ALBUMINOID HUMOR

PUPIL

Figure 2.1. The eye according to Hunayn ibn-Ishaq (Lindberg, 1976).

22 Ibn Al-Haytham (Alhazen)

Ibn al-Haytham (c. 1000), the Arab scholar who made the greatest contribution to visual theory in his time, was a dedicated self-taught man. As was generally the case at that time, the information which he collected was comprehensive. He was interested in theology, medicine and philosophy, but chiefty in physics, astronomy and mathematics. His output was enormous: 60 works on mathematics, physics and astronomy and the same nu mb er on theology; also books on geology, calligraphy and medicine. Alhazen's most important work is the voluminous work on optics Kitab al-Manazir, which later became known in the West as De aspectibus or Perspectiva. In this book he writes not only on reftection and refraction, but also on the theory of vision [6]. Alhazen's theory ofvision In agreement with Aristotle, Alhazen's point of view is that vision is a passive experience. He thus distances hirns elf decisively from Euclid and Galen. At the same time, however, he maintains the geometrical basis of the extramission theory with the following argument: every point on a colored object radiates diverging bundles of light rays in many directions, some of which fall on the eye. It is thus not 'forms' which leave the visual object but each individual point reaches the eye by means of light rays. Instead of the holistic 'form' concept of Aristotle, he propounds the atomistic concept of lightradiating points. Everything visible in the visual field sends out rays which enter the pupil and come together in the middle of the eye. Alhazen thus simply reverses the direction of the rays in the Euclidian cone. Alhazen has discovered by studying the camera obscura, the pinhole camera, that rays from various directions can meet at one point without colliding with each other. Nor do colors lose their identity in these circumstances. Alhazen takes another important step forwards and, in so doing, becomes the first practitioner of physiological optics: he states that every point in the outside world is represented in the lens, the organ of sight. With this statement optics has for the first time entered the eye and the first concept of an 'image' of the outside world in the eye has been presented. It is not easy to prove such astatement, and Alhazen finds several artifices necessary. The cornea is struck by rays coming from all directions; how can the eye distinguish which rays come from which direction? Alhazen's answer is that only unrefracted rays playapart in vision. Unrefracted rays are rays which fall at right angles onto the cornea and the front of the lens. Because his theory would otherwise be disproved, Alhazen assurnes that the curvatures of the cornea and the front surface of the lens are spherical and concentric. If the ray strikes the cornea at right angles it will do the same for the lens. The visual process, as conceived by Alhazen, is represented in Figure 2.2. There is point-to-point projection

23

OBJECT

VITREOUS

HUMOR

Figure 2.2. The geometry of sight according to Alhazen. The image of the object in the lens

is formed by the non-refracted rays from each point of the object (Lindberg, 1976).

of the visual object onto the posterior surface of the lens. But what about the rays which do not strike at right angles? According to Alhazen these are of little importance: the right angle is the ideal angle. An arrow directed straight at the victim is more dangerous than a glancing shot!

24 The visual information goes straight from the lens to the optic canal. Naturally the rays may not continue to progress in a straight line; they would then, as in the camera obscura, cause inversion of the image when they have passed the convergence point in the middle of the eye. Alhazen again has resource to an artifice: the rays which leave the lens are parallel. Refraction, which was ignored at the passage from air to cornea, now seems to appear as a deus ex machina by the passage from the lens to the vitreous. The points of the image now retain their spatial arrangement, and this remains so until the image, via the hollow optic nerve and the chiasma, has reached its final destination in the ultimum sentiens (Fig. 2.3). The passage through the nerves is naturally no longer an optic process; the lens - as organ of perception - is the place where a 'visual image' is made from the rays. The transportation of that image through the vitreous is thus already more than a purely optic process. Exactly

Figure 2.3. The visual system according to Alhazen. The hollow optic nerves meet in the chiasma and then diverge (Lindberg, 1976).

25

where the last station of sensory perception is situated is uncertain; in any case it is not peripheral to the chiasma, where the combination of the two images, one from each eye, takes place. With Alhazen a new era has dawned in the science of vision. He has conceived a theory in which image-forming takes place in the eye, in such a way that every point in that image corresponds with a point in the outside world. Although he breaks with the theory of extramission, his new theory retains the geometrical qualities of the old one. The fact that none of the extremely acute scholars who lived before Alhazen had come this far, shows that his theory was an exceptional intellectual achievement. He found his way like a sleepwalker without awakening from his dreams. They were the dreams of his time: that the lens was the seat of vision, and that the outside world could not enter our ultimum sentiens upside down. It would take six centuries before science awoke from these dreams. Calors Alhazen's doctrine of color perception, which shows the inftuence of Aristotle and Ptolemy, is empirical. Light and color are the only primary visual information. Perception is achieved by comparing this information with what we know already. This comparison takes place so quickly that we are not aware of the process. (Helmholtz speaks later of 'unconscious inferences'). Alhazen writes: Therefore, that which sight perceives by pure sensation is color qua color and light qua light. Nothing else is perceived by pure sensation, and all properties other than these two can only be perceived by discemment, inference and recognition ... Thus perception of color qua color precedes perception of the quiddity of color, the latter being achieved by recognition [6].

The psychology of color vision receives Alhazen's attention. Like Ptolemy he carries out tests with a rotating wheel with colored sectors. He calculates the minimum perception time from the blending of the colors. He also studies the effect of different backgrounds on the appearance of colors. Alhazen is the first to describe 'colored shadows' (p. 120). When the sun shines on a green meadow, in the shadow on a white wall a green color can be seen; the light which is reftected from the grass is accompanied by the color. Alhazen, like Aristotle, distinguishes true colors and apparent colors. Most of the colors of animals are true, but some colors change with the direction of gaze, like the colors in a pigeon's neck. (The difference between true and apparent colors will intrigue people for centuries to come. For the time being no one doubts that the colors of the rainbow are apparent, but about the colors in the pigeon's neck every medieval opticist has his own opinion).

26 The refraction of light Alhazen gave an explanation for the phenomenon of refraction [7]. Unlike Aristotle he considered that the velocity of light was finite, and he assumed that light was slowed down in a more compact medium. He asked himself why a ray of light falling at an angle out of the air into water is be nt towards the vertical, and gave the following answer: because the horizontal component of the ray is slowed down more than the vertical component. (Again the slanting rays have to suffer, like the oblique rays which fall on the cornea). Alhazen studied not only the refraction at a flat surface but also the refraction in a globe filled with water. In that case it is not possible that he did not notice the dispersion of colors, and it is all the more remarkable that the great master of medieval optics hardly mentions the colors of the rainbow in his Kitab al-Manazir. Later Arab commentators of Alhazen realized that the glass globe was an ideal model of a raindrop, and that the rainbow was caused by refraction and internal reflection in a drop of rain. Kamal al-Din al-Farisi (died c. 1320), Alhazen's principal commentator, produced in the first decades of the fourteenth century the correct geometrie optic explanation of the colors of the rainbow [8], at the same time as, but independently of, the scholastic opticist Theodoric of Freiburg (Fig. 2.7) and long before Descartes (Fig. 4.13, from [9]). The main rainbow is formed by two refractions and one reflection, and the secondary rainbow by two refractions and two reflections. The colors, however, remain a mystery for Kamal. Kamal al-Din disagreed with the distinction made by AristotIe and AIhazen between permanent and apparent colors. The colors of objects, considered to be permanent, differ in moonlight, in sunlight or in firelight. This view was correct, but it was not accepted before the time of Boyle and Newton. It is worth pausing here to consider the fact that optics at the time of Kamal al-Din had reached the same level in the East as in the West. Why did Western knowledge develop further at ever increasing speed? Was that the result ofthe institutionalization of western learning in the universities? Or was it the early link between science and technology? Perhaps it was merely a question of chance: the islamic civilization was unlucky enough to be overrun by barbarian hordes; in 1258 Bagdad fell into the hands ofHulagu Khan. The West was spared such a disaster by the sudden death of the Great Khan in 1241 [10].

27 The science of vision and colors in the prime of the Middle Ages In the eleventh and twelfth centuries there was an important political, social and cultural renaissance in Western Europe. The efficient use of waterpower led to an 'industrial revolution'. This resulted in a marked increase in population, urbanization and the founding of schools. The existing cloister schools were supplemented by city schools, often connected with cathedrals. This development was soon followed by universities, the first in Bologna in 1150, subsequently in Paris, Montpellier, Padua, Oxford, Cambridge and many other cities. The rapid growth of intellectual life opened western culture to infiuences from other civilizations, the Byzantine and especially the Arabian. The infiux of Arab learning occurred mainly between 1150 and 1250. Constantinus Africanus (c. 1025-1087) is one of the heraids of 'arabism' . After an adventurous life in Africa and the East he takes up translation in the monastery of Monte Cassino. In particular, he translates Galen from the Arabic. Thus it took nearly a thousand years for Galenic medicine and physiology to make its way from Rome, with adetour through Arabia, to Salerno. He also translated Hunayn Ibn lshaq's 'Ten Treatises'. An important center of translation was Toledo, an Arab city which fell to Castille in 1085. Alhazen's optic work was translated here under the titles De aspectibus and Perspectiva (c. 1170). The introduction of Aristotle's complete works (c. 1270), translated directly from the Greek, was very important. Many theologians, among whom Albertus Magnus (1200-1280) and Thomas Aquinas (1224-1274), devoured Aristotle's books. They came to the conclusion that it was possible to combine harmoniously Aristotelian natural science with a Christian theology which was philosophically formulated in the Aristotelian manner. Thus Aristotle became a sort of John the Baptist, the precursor Christi in naturalibus. With all this, philosophy naturally still remained the handmaiden of theology. William of Conches (1080-1145), a French scholastic who has written a famous commentary on the Timaeus, thus a Platonist, considers, following in Galen's footsteps, that color vision is brought about by an interior ray emitted through the optic nerve and the pupil, which mingles with the external light, 'sensing' the color of the object seen and returning to the eye and the soul carrying the information of color. An important argument in favor of 'extramission' is the existence of the evil eye, which may fascinate people, often to their disadvantage. Like all previous authors (see quotation from Alhazen on page 25), William of Conches also considers color to be the specific visual information. For hirn and later authors, visual impressions are assembled from color, the

28 'proper sensible', and the common sensibles in the sensus communis. Alhazen named twenty such 'visible intentions' which act on our interior senses: size, distance, texture, but also beauty and ugliness. William of Conches describes once more the process of visual perception, together with its neuro-anatomical basis [11]. Following Hellenistic practice, there are three consecutive, connected cavities in the brain (Fig. 2.4). The front one is the cellula phantastica where all the information from the senses is assembled and where the sensus communis is situated. The second one is the cellula logistica, where the sensory impressions are logically ordered. The last cavity is the cellula memorialis, the place where information is stored. Of course there is two-way traffk between the two latter cavities: consultation with the memory is often necessary for the logical construction of a perception. Robert Grosseteste (1168-1253) is an important transitional Figure [12]. He was the first Chancellor of Oxford University and later Bishop of Lincoln. He combined the older Augustinian Platonism, according to which a rational explanation of the world could be provided by mathematics, with the empiricism of Aristotle. Characteristic of hirn is his metaphysics of light, deriving from Plotinus and Augustine (who had spoken of God as 'infinite incorporeal Light'). Grosseteste believed that Light was the first incorporeal form, that is, the substantial basis of spatial dimensions and the first principle of motion, and that the laws of Light were the basis of the scientific explanation of the physical world. He explains his cosmogony as follows: Light, which is the first form created in first matter, multiplied itself by its very nature an infinite number of times on all sides and spreaded itself out uniformly in every direction. In this way it proceeded in the beginning of time to extend matter which it could not leave behind, by drawing it out along with itself into a mass the size of the material universe [l3]. It is a passage which will remind some readers of the 'big bang' and the expanding universe. Older readers will think of Hegel's theory, that 'light is infinite generation of space'. Georg Friedrich Wilhelm Hegel was one of the last metaphysicians of light. Grosseteste proceeded to make a detailed study of optics. Optics was the highest form of natural science because experientia and mathematics met there. The great scientific interest in optics in the Middle Ages had its origin in light metaphysics. Robert Grosseteste wrote a theory of vision. He wrote about light and colors, about geometrical optics and the rainbow. The rainbow is not the result of reflection of the sun's rays from spherical raindrops, but of refraction of

29

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Figure 2.4. The three brain cavities. The anterior cellula phantastica contains the sensus communis. The second cavity is the cellula logistica: the third the cellula memorialis (Gregorius Reiseh, 1486).

the rays. As far as vision was concerned he still believed in extramission. He considered light to be an instantaneous succession of waves. He examined the colors which occur on refraction in prisms and in glass globes filled with water. He thought that the colors produced were caused by differential refraction and by different degrees of darkness, colors being produced by weakening of

30 white light. Grosseteste formulated a law for the refraction of light in water: according to hirn the angle of refraction is half the angle of incidence. The perspectivists Not without reason,Grosseteste has been called a transitional figure. Color and light theorists who lived a few decades later could draw on new sources of knowledge: the complete works of Aristotle and the Perspectiva of Alhazen. The three following scholars, Bacon, Witelo and Dietrich of Freiberg, were Aristotelians - but still retaining a certain affinity for neo-platonism - who chose Alhazen's Perspectiva as the starting point of their optic studies, and have therefore been called 'perspectivists' [14] . Bacon was the pioneer of 'perspectivism', Witelo was an optieist who had great influence, extending even to Kepler; Theodoric of Freiberg, the last of the three, formulated a theory of therainbow which antieipated Newton by more than two centuries. Roger Bacon [15] (1214-1292) studied and taught at Oxford and Paris. He was an expert on the scientific work of Aristotle and the Arab scholars. Following in Robert Grosseteste's footsteps, he specialized in scientia experimentalis,and is considered as the prophet of modem natural science and technology; he foresaw telescopes, automobiles and airplanes. But the word 'experimental' was widely applied at that time, and included religious experience, magie and astrology. Bacon applies hirnself not only to theoretieal but also to practical optics. He studies refraction by letting sunlight fall into water-filled globes (corpora urinalia). In his imagination he sees immense incendiary mirrors able to set enemy camps on fire. Bacon has no original ideas about colors. He rejects the idea that the rainbow is formed by refraction, because it moves with theobserver and 'must' therefore be due to reflection. Bacon considers that both the colors in a prism and the colors in a pigeon's neck are true colors. Bacon's theory of vision is based on Alhazen's book. He also discovers numerous arguments to show why the rays falling at right angles on the cornea and lens bring about vision. The perpendieular ray is shorter, and thus stronger. A diagram of the eye and the path of the rays is shown in Figure 2.5. In his treatises on light and color Bacon tries to combine Aristotle (color and light as form without material) and Alhazen (color and light as radiation from all points). He considers (in the manner of the neoplatonist AI-Kindi) that all things radiate powers in media which are receptive to them. Thus the magnet sends outpowers, species, which act on receptive iron. This also applies to the sun, which surrounds itself with a field of power, in this case light-species. This is not a material emanation, otherwise the sun would have burnt out long ago! It is rather that the sun, in the Artistotelian sense, actual-

31

Figure 2.5. The path of light rays according to Roger Bacon. a-l-b is the pyramid pictured in Fig. 2.2; c-d is the pupil. The white disc in the center is the lens.

izes the potential of visual objects to be luminous. The illuminated object passes the species on to the adjacent medium, which passes it on to the next, and so on. This is the multiplicatio specierum, by means of which light spreads in a sort of relay race. The multiplication is not a flow of matter like water, but a kind of pulse propagated from part to part. Light can only multiply in a medium; space is therefore a plenum, a vacuum does not exist. (The reader may notice the resemblance between the multiplicatio specierum and Huygens' principle). The species enters the eye and multiplies itself tortuously in the tortuous optic nerves, without the arrangement of the species being disturbed. In this way an image of the outside world (naturally the right way up) is formed in the sensorium. In the old conflict between the two aspects of vision - active or passive - Bacon's train of thought is complicated. On the one hand, as follower of Alhazen, he is a convinced intromissionist. But at the same time he is an extramissionist in his conviction that the multiplicatio specierum comes from twodirections. Multiplication of visual power also goes in the opposite direction. This is far from unnecessary because it is only in this way that the rays of light acquire the dignity needed to produce visual perception; following the precept of Augustine: the soul may never be subordinate to matter, Bacon wrote:

32

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and astronomy at lngolstadt and writer of the book Oculus: hoc est fundamentum opticum. (1619). He had seen in Rome how the opaque coats were dissected from the back of ox's eyes so that the inverted retinal image became visible. (It is curious that such an experiment was performed as early as 1595 by the anatomist Aranzi [13].) But another Jesuit, Grimaldi, did not even mention Kepler in his important work (1665) on light and colors (p. 59). Kepler's theory was slow in reaching the medical world. Boerhaave (16681738) was the first professor of medicine to expound Kepler's theory to his

49 students. It was not until 1708 that the Academie Royale des Sciences in Paris formally conceded that 'it was possible to see without a lens, the organ which had always been considered as the primary instrument of vision' . After two thousand years of visual theory, Kepler was the first to make it clear how the retina could function as the seat of vision. Centuries more were necessary before the function of the retina was further elucidated; that knowledge will not be considered in this book until Chapter XII. The retina was usually, after Kepler, regarded as the organ of vision, but the theory of color vision rose to great heights in the centuries after Kepler without noticeable insight into the anatomy and physiology of the retina. Kepler called his 1604 book Ad Vitellionem paralipomena, appendix to Witelo. He thus indicated that he considered that his work completed the perspectivistic tradition. That is a very modest assertion from someone who had repudiated Galen's and Alhazen's theories. In reality Kepler is more of a revolutionary. He is one of the most important initiators of a movement which disposed of the whole range of medieval science. This movement is called the 'scientific revolution' [14].

IV Light, color and vision during the scientific revolution

The scientific revolution Kepler and Galileo In the history of the natural sciences the 'scientific revolution' begins around 1600 and ends with Newton's theory of gravity, which marks the beginning of classical physics (1687). One book that was to revolutionize natural science had been written fifty years earlier: Copernicus' De revolutionibus orbium coelestium (1543). But for most of Copernicus' contemporaries this book was nothing more than a mathematico-astronomic philosophy, simpler than the Greek system of Ptolemy. At first very few recognized in it a complete revolution in man's concept of the world. Johannes Kepler was the first astronomer to take Copernicus' heliocentric system really seriously, and thus to believe that the earth was just a planet among other planets. Kepler's work was based on the extremely precise measurements of his Danish master, Tycho Brahe. A discrepancy of 8 minutes of are between observation and calculation forced Kepler - as he hirnself writes - after years of reckoning, to a complete revision of astronomy. Kepler is thus the creator of the 'precision universe' in which observation and mathematics forms a close alliance. Galileo Galilei (1564-1642) is the other principal character at the beginning of the scientific revolution. He is also enthralled by the mathematical order which mIes in the book of nature. In The Assayer (ll Saggiatore, 1623), he writes: Philosophy is written in this grand book, the Universe, which stands continually open to our gaze. But the book cannot be understood unless one first learn to comprehend the language and read the letters in which it is composed. It is written in the language of mathematics, and its characters are triangles, circles and other geometrie figures without which it is humanly impossible to understand a single word of it; without these, one wanders about in a dark labyrinth [1]. But like Kepler, Galileo differs from Platonists and Pythagoreans in that he not only uses mathematics, but also observation and experiment to help hirn

51 in his attempt to comprehend the order of nature. Galileo lays the foundation of mechanics and defines the methodology of physics: isolation by experiment of simple, mensurable processes and subsequent deduction of general, mathematically formulated, laws. Like Kepler, Galileo breaks radically with Aristotle's geocentric view of the world, and also with his obsolete theory of motion. He also differs from Aristotle on another essential point: Galileo's interest is quantitative, not qualitative. For this reason, his ideas move in the direction of atomism. He adopts Democritus' idea that sensory qualities do not belong to the objective world but are produced in the subject. As in Il saggiatore: To excite in us tastes, odors and sounds, I believe that nothing is in us except shapes, numbers and slow and rapid movements. I think that if ears, tongues, and noses were removed, shapes, numbers and motions would remain, but not odors, tastes or sounds. The latter, I believe, are nothing more than names when separated from living beings [1]. Although Galileo did not explicitly occupy hirnself with colors this idea had important consequences for the theory of color. Bacon, Gassend and Descartes The fall of Aristotelianism called for a complete revision of thought. Three men assumed this task: Francis Bacon, Rene Descartes and Pierre Gassend. It is the irony of history that, in the ideas of the first two of these, the alliance between mathematics and observation, which had been so fruitful in Kepler's and Galileo's work, was broken. Bacon became the prophet of observation and experiment and neglected mathematics. His inftuence was greatest in the 'Baconian sciences', such as natural history and chemistry, which were still far removed from the mathematical stage. Descartes was in search of a mathesis universalis and neglected empirical observation. He was the great theorist of the mechanical sciences. Gassend was a philosopher who, although he did not contribute much to either the empirical sciences or their mathematical elaboration, nevertheless, played an important part in shaping modern science. Francis Bacon (1562-1626), Baron of Verulam, Viscount St. Alban, Lord High Chancellor of England, need not be treated in detail in this book. He was a champion of anti-aristotelianism and provocatively called his book, which was meant to take the place of Aristotle's Organum, the Novum Organum. Although he was not a creative scientist hirnself, he contributed largely to the development of empirical science in England. Although Bacon did not consider that the collection of facts was the final goal of science, he certainly thought that that was its most important function. His inspiration came from practically acquired knowledge, such as alchemy and hermetic science. His

52 ideology was utilitarian and he realized that science was helpful to technique: in the brewery, and the manufacture of gunpowder, paper, and mechanical and navigational instruments. In the century in which England with its voyages of discovery was beginning to become a world power, Bacon's favorite maxim was: 'Nature to be commanded should be obeyed'. Pierre Gassend (1592-1655), canon of Dijon, wrote an anti-aristotelian pamphlet in his youth: Exercitiones paradoxicae adversum Aristoteles. He performed the feat of making Democritus' materialistic teaching (and even the moral philosophy of Epicurus) acceptable to the ecclesiastical authorities. He was an admirer of Galileo and believed that the physical world consisted of atoms, which were sometimes aggregated to form 'molecules' of various weights and sizes, particles which obeyed the laws of mechanics in empty space. Gassend made a sharp distinction between physical matters on the one hand, and supersensory matters on the other, over which only the church and revelation had authority. Rene Descartes (1596-1650) contributed largely to the shaping of modern science. Mathematics played a dominant part in his thinking. Descartes even believed in a sort of metaphysical mathesis universalis and considered that natural science, like mathematics, could largely be a matter of deduction. This point of view makes Descartes the most important representative of rationalism. Descartes' view of nature is dualistic, because he makes a sharp distinction between body and mind. The essence of the mind is thinking: cogitatio: the essence of matter is magnitude: extensio. Space is a plenum, completely filled with material particles of various shapes and sizes. Man alone has selfconsciousness and amind. The inanimate nature and lower forms of life obey the laws of mechanics. The basis of everything that happens in nature is movement of particles. Descartes' corpuscular theory differs from Democritus' atomism, in so far as Descartes denies the existence of a vacuum and thinks that matter is infinitely divisible. Following the example of Democritus and Galileo, the qualities, like scent, touch and color, are disregarded. Only objective phenomena in space and time remain of interest to science. As a rationalist who was hardly impeded by the censorship of experiments, Descartes was able to construct a fantastic system of nature. Little of this has remained intact. His lasting farne derives from his invention of analytical geometry and his mechanistic theory of natural science, including physiology. As physio!ogist, Descartes thinks in corpuscular terms [2]. He therefore rejects Galen's teleological line of thought and his theory of the four body fluids. He does not accept any of the animi sm of the Stoics. In Descartes' time, the soul was usually credited with two functions: in the first place, it was the principle of life and, in the second place, the conscious principle

53 of perception, will, thought and feeling. The crux of Descartes' physiology is that he rejects the soul as the principle of life. He adopts Galen's pneuma theory in a simplified and materialistic form. He believes in the flow of animal spirits; these come from the blood and go to the cavities of the brain. From there they pass through the brain substance and go to the hollow nerves of the brain and spinal cord. The width of these nerve canals can vary: under the influence of stimuli they open wide to allow the passage of animal spirits from the brain cavities. In this way the reflex arc is established which is shown in Figure 4.1. An important and highly original feature of Descartes' thesis is that the origin and association of ideas are connected with material occurrences in the brain. With 'ideas' Descartes had in mind sensory sensations and the passions of the soul. That is a fundamental concept which is still maintained today. In a more generalized sense, it was reformulated by Mach in 1865 in the 'heur-

Figure 4.1. The reflex are aeeording to Deseartes.

54 istic principle of psychophysical investigation': every psychic occurrence is accompanied by a physical one [3]. As ideas, on the one hand, belong to the immaterial order of cogitation and, on the other hand, are represented by material processes, there must be a place somewhere in the brain where mind and body meet and exert influence on each other [4]. According to Descartes, this is the conarium, the pineal body. It seems to be the obvious place for contact between mind and body. It is small and therefore an appropriate seat for the immaterial and spaceless mind; it is also singular and thus a suitable meeting-pi ace for the sensory impressions from the two eyes, the two ears and the other senses; in short it is a suitable organ of 'common sense', sensorium commune. Descartes also thought that the pineal body could move: with light taps it could open and dose pores in the lining of the cavities of the brain. Thus the mind 'plays' the brain like an organist plays his organ.

Descartes and vision Kepler's theory of the inverted retinal image went against reason and was difficult to reconcile with the opinions of the time. His theory conflicted with the teaching of Aristotle and many scholastics; according to them the shapes or species, life-like images of the outside world, found their way into the senses. With such a theory, an inverted retinal image would have to produce an inverted picture of the world. An attempt was made to solve the problem by assuming that the human mind, looking at the retinal image from the inside, was able to turn it right side up again. But this hypothesis of a homunculus, a little man in your head, soon stranded. Because who looks at the homunculus' retinal image? If this line of thought is followed, a succession of smaller and smaller homunculi appears, like aseries of Russian dolls, fitting one into the other. Kepler hirnself tried to solve the problem with his projection theory (p. 45), but this attempt was doomed to failure because it involved visual rays. These no Ion ger fitted into the modern line of thought. The inverted retinal image (Fig. 4.2) made a new theory of vision necessary. This was provided by Descartes, the central figure in the transition from medieval scholasticism to the natural science of the new era. Descartes' theory of vision is mechanistic (Fig. 4.3). The fibers of the optic nerve, which also follow separate paths in the chiasma, transport the mechanical changes produced by light in the retina. The fibers work like taut ropes: if you pull on one end, that has an effect on the other end. In this way a double representation of the outside world is obtained in the brain, one from each eye. With the help of the animal spirits in the cavities of the brain, the data from the two representations are combined in the sensorium commune.

55

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56

Figure 4.3. Vision according to Descartes. Each retina projects onto the wall of the brain cavity. The projections are united in the pineal body (Descartes, 1664).

The internal representation of the retinal image has not much resemblance to the visual reality; the cerebral image differs from the outside world. Descartes writes about the cerebral images as folIows: We must at least observe that there are no images that must resemble in every respect the objects they represent ". but that it is sufficient for them to resemble the objects but in a few ways, and even that their perfection frequently depends on their not resembling them as much as they might ." You see that engravings, being made of nothing but a little ink placed here and there on the paper, represent to us forests, towns, men and even battles and storms, even though among an infinity of diverse qualities which they make us conceive in these objects, only in shape is there actually any resemblance. ". Now we must think in the same way about the images that are formed in the brain, and we must note that it is only a question of knowing how they can enable the mind to perceive all the diverse qualities of objects to which they refer; not of knowing how the images themselves resemble their objects [5]. This is an astounding passage: it is centuries later that the problem of selective codification of contours will be tackled again, and a 'primal sketch' will be regarded as the first phase of visual identification (p. 238). In animals, creatures without a soul, the visual signals are integrated in the sensorium commune with signals from other special senses and data from memory. Descartes considers that the 'body memory' is situated in the outer convolutions of the brain; the impressions of earlier events are stored there (Fig. 4.4). After the above process of integration and association has taken place, an impulse goes out from the brain to the musdes, through which the 'reflex' is put into effect.

57

Figure 4.4. Descartes' conception of memory (1664). How 'impressions' are made in the brain substance.

In human beings, endowed with amind, the coded signals are decoded in the pineal body and translated into sensory perception. There some whirls of particles become color, others become sound. Descartes does not try to explain perception on the basis of mechanical events; it has been created by God's Providence. Descartes' theory of visual perception influenced conclusively ideas on this subject. His opinion that no exact copy of the outside world is made in the sensorium commune, but a different coded description, was entirely new. This idea made the inverted position of the retinal image an unimportant physiological detail, nothing more than a first sign of the different nature of the physiological process. In Figure 4.3 Descartes puts the representation of the arrow in the pineal body upright, but motivates this with the words 'for example'. In other words, the arrow might just as weIl have remained upside down; it is wasted effort to look for an exact copy of the outside world in the physiological process. Another new idea is that the nerves are only channels for the conduction of unspecific, mechanical signals. The specific character of the optic nerves is not determined by the specialized nature of the conducted signals, but by the fact that they come from the eye and end at the cerebral representation of the visual world. Two hundred years later, this fact was to be rediscovered by Johannes Müller as the 'law of specific sense energies' [6], and, in this century, by Rushton [7] as the 'law of univariance'.

58 The modem reader of the Traiti de l'homme will be surprised by the detailed descriptions (with illustrations) that Descartes gives of physiological processes which have absolutely no basis of physiological observation and are purely products of fantasy. In this context, Voltaire said that Descartes would have been the greatest philosopher in the world if he had not invented so much. But Descartes was not interested in facts. His object was to design a model of a human robot which could function without the intervention of the soul. New theories of light and color

The supporters ofthe corpuscular theory all agree that light is not an accidens, a condition of something else, but a substance. That means that they also have completely new ideas about the relationship between light and color. Light and color had always been two totally different things. For Plato, who still believed in visual rays, color was a property of things and not of the eye. In the peripatetic philosophy light and color were also different. A problem for the Aristotelians was the 'apparent' colors, colors which seemed to appear from nowhere - in the absence of colored objects - and should not really exist: the colors of the rainbow, prismatic colors, the colors in a pigeon's neck. In contrast with the material colors, the permanent colors of objects, in short: the true colors, the apparent colors were called c%res fugitivi, phantastici, emphatici even colores fa/si. When people began to regard light as a substance, the idea arose everywhere that there must be a close connection between color, whether true or apparent, and light. But the nature of the connection was difficult to define. Beeckman and Gassend sought it in the form and position of the light particles, Descartes in their rotation. The ideas of Grimaldi and Hooke were more directed towards various types of vibration, while Huygens had to admit that he did not know the answer. Isaac Beeckman (1588-1637), rector of the Latin school in Dordrecht, is one of the first seventeenth century scientists to consider light as corpuscular, indeed one of the first proponents of the mechanicistic philosophy [8]. In his thesis (Caen, 1618) he already makes the assertion that light consists of minute particles of material moving in empty space. He rejects the species theory of the scholastics and thus considers that there is no difference between true and false colors. The different colors are produced by reftection of light from material with pores of various sizes and shapes. The light which is altered in this way can stimulate the retina in various ways and thus produce different sensations of color. Beeckman compares music and color. If transparency is the keynote, then yellow is the dominant, blue the minor third, red the major third, green the

59 fourth and white the octave. In this linear color scheme the absence of black, the indispensable peripatetic ingredient of colors, is new and remarkable. The relationship between sound and color, which had already been studied by Aristotle, continues to arise in the discussions on color until 1800. Beeckrnan did not publish much in his lifetime, but he had good contacts in the world of scholarship and probably had considerable inftuence on Gassend and Descartes. Gassend (1592-1655), who was inftuenced by Beeckman, believes in light particles moving at great speed through empty space. Colors are produced by the shape and position of the light particles. If tartaric acid is added to a colorless extract of sen na leaves the liquid becomes red; red can thus apparently arise from non-red material by mixing and alteration in the position of the light particles. This color is both fugitive and - unlike the colors in the pigeon's neck - independent ofthe direction of gaze. It seems that there is no real difference between true and apparent colors: both arise from light. The right proportion of light and darkness in the various colors is too great a secret for man to be able to capture it in a hypothesis. Descartes thought that light was propagated at infinite speed in a spatial plenum of particles of various sizes, like an impulse in a mass of inelastic particles is propagated without noticeable displacement of the particles. There was only one possible mechanical option to explain color: if light is propagated by a linear impulse, color must arise through rotation of particles [9]. To give a popular (but not very consistent) illustration of his theory, he compared light to tennis balls which, when hit by the racket, not only are given a direction, over the net, but also rotation, which may even cause them to change direction. He tried to make his theory more credible by referring to the prismatic colors and the colors of the rainbow. He thus tried to explain the colored edges of a ray of light passing through a prism by assuming that the stationary ether particles in the shadow were made to rotate in one or other direction by the ether particles which had been set in motion by the light. This hypothesis is still not far removed from Aristotle's idea that color arises by the meeting between light and darkness. When Descartes looked through a prism at a white surface, he saw colors at the edges: new evidence for the importance of darkness for the appearance of colors. In this way he fell into the same trap as, much later, Goethe was to do, who saw the colors at the edges as evidence of his Aristotelian color theory. Francesco Maria Grimaldi (1628-1663), a scholarly Jesuit in Bologna, wrote in his short life a long book, entitled Physicomathesis de lumine, coloribus et iride, about light, color and the rainbow. It was published posthumously in 1665 and describes an important discovery: the diffraction of light. Grimaldi is the first to do experiments with very small spots of light. He

60

Figure 4.5. Diffraction (Grimaldi, 1665). In the case of rectilinear propagation, light passing through two holes would illuminate the surface between N and O. Light at I-N and O-K is due to diffraction.

examines the shadow of an object illuminated by a pinpoint source of light. The shadow is not only bigger than expected on geometrical grounds (Fig. 4.5), but inside and outside the shadow lines are seen at regular intervals, aIternately bordering on zones of red and blue. Grimaldi concludes that light is not only propagated in straight lines, by reftection and by refraction, but that there is a fourth form of propagation: 'diffraction'. (This is a term which is still in use). Grimaldi asks himself whether light consists of waves; he compares the light which can bend round an obstacle with the movement of waves in water: there too a movement can continue round an obstacle. Grimaldi thinks that colors are different sorts of light waves, just as notes consist of different sorts of sound waves. However, he does not manage to elaborate his wave theory further, and, as an obedient Jesuit, finally accepts that light is an accidens. According to Grimaldi there are single and compound colors, just as there are single and compound musical notes. He thinks there are three single colors: red, yellow and blue; like Aguilonius, he propounds a trichromatic color system. Grimaldi's discovery that colors can arise from light without the intervention of any device such as a prism (sine mutatione medii), is essential to the theory of color. Color is a property, or at least a modification, of light

61 itself. The distinction between true and false colors is therefore, according to Grimaldi, unnecessary. Robert Hooke (1635-1703) was an English scientist of great experimental proficiency. He studied with the famous Robert Boyle, England's leading natural philosopher, and later became his assistant. When the 'Royal Society of London for improving of Natural Knowledge' was founded around 1660, Robert Hooke was appointed Curator of Experiments. Hooke's most important work is the book Micrographia, which was published in 1665. It is the first large book about the microscope. The microscope is described in detail, together with all sorts of objects that can be seen with it, illustrated with beautiful engravings. The book also includes the first systematic study of colors in thin layers, as in 'Muscovie Glass' (mica), soapbubbles, very thin blown glass and glass plates pressed together. When the thickness of the layer of air between two glass plates varies, colored rings appear; the colors are in the same order as in the rainbow. Hooke constructs a new theory of light and colors on the basis of his findings with colors in thin layers. He assumes that light is propagated at a finite speed and that it consists of a rapid vibration. To his mi nd it is impossible that light should consist of the transportation of particles. When a diamond is rubbed it remains luminous for a short time in the dark, but it does not decrease in size. In view of the extraordinary hardness of a diamond, the vibration of light must be extremely rapid. Hooke also tries to visualize the vibration in a light bundle which is refracted at the surface of a denser medium, 'a medium that yields an easier transitus to the propagation of light'. Figure 4.6 [10] gives a schematic representation of this situation. Hooke thinks that wave fronts occur in white light at right angles to the direction of propagation of the light. After refraction oblique wave fronts occur, which give rise to red or blue according to their position. Red and blue are thus the two primary colors, all other colors are built up from these two. Hooke also applies this hypothesis to colors in thin layers (those which his teacher Boyle one year earlier had investigated) and asserts that oblique wave fronts form the basis of all colors. These considerations are capable of explicating a11 the phenomena of colours, not only of those appearing in the prisme, Wate r-drop , or Rainbow, and in laminated or pated bodies, but all that are in the world, wether they be fluid or solid bodies, wether thin or thick, wether transparent, or seemingly opaceous [11]. The appearance of colors in thin layers shows that colors cannot be the result of refraction: This experiment therefore will prove such a one as our thrice excellent Verulam calls experimentum crucis, serving as a Guide or Landmark, by

62

Figure 4.6. Hooke's hypothesis (1665) that color dispersion is caused by the oblique orientation of the wave front after refraction (1665) Boyer, p. 235.

which to direct our course in the search after the true cause of Colours [12]. Three years earlier Hooke's great riyal, Isaac Newton, had proven in another experimentum erucis that refraction was the 'true cause' of colors. Like Grimaldi, Hooke arrives at a sort of wave theory of light on the grounds of color phenomena. He certainly deserves credit for the introduction of the concept of wave fronts. But his hypothesis is rather conjectural and not mathematically formulated. When Huygens gave a mathematical foundation to a wave theory, it proved unable to explain colors. Christiaan Huygens (1629-1695), scion of a prominent and cultured Dutch family, spent the middle period of his life in Paris, where he was joint founder of the Aeademie des seienees. He was considered the greatest mathematician and physicist in the years between Galileo and Newton. Descartes' mathematical view of nature was elaborated by Huygens with every available mathematical technique. He explained every natural phenomenon by collisions between partides: particles of matter, ultrafine balls of ether and gravitational bodies. He is best known for his mathematical analysis and technical improvement of the pendulum mechanism of docks: Horologium oseillatorium (1673). His other well-known work is the Traite de la Lumiere (1691).

63 Huygens conceived his wave theory of light in about 1672-1673, but delayed its publication. He agreed with Descartes (in disagreement with Gassend and Newton) that light cannot consist in the transport of bodies from the luminous object to the eye. Unlike Descartes, he thought that light was not an instantaneous impulse to movement but a disturbance of equilibrium propagated in time and space. For this reason, he considered ethereal material to be elastic. R0mer's discovery of the speed of light gave hirn support here. In Huygens' theory of light lines are cut by spherical surfaces representing successive loci of a central disturbance. Every particle of the luminous body (Fig. 4.7) must be regarded as the center of its own spherical 'wave' (an aperiodic, isolated impulse). The essence of 'Huygens' principle' is the idea of secondary waves; on this subject he writes: that each particle of matter in which a wave spreads, ought not to communicate its motion only to the next particle which is in the straight line drawn from the luminous point, but that it also imparts some of it necessarily to all the others which touch it and which oppose themselves to its movement. So it arises that around each particle there is made a wave of which that particle is the centre [13]. (see Fig. 4.8. Wave DCEF comes from A, but from B a wave also arises, KCL, which contributes to the total effect). The object of Huygens' hypothesis was the mathematical formulation of Descartes' light theory. A Baconian inventarisation of phenomena associated

Mais il faut confiderer encore plus particulierement l' origine de ces ondes ,& la maniere dont elles s'eftendenr. Et premierement il s'enfuit de c~ q ui aeIl:e dit dela prod uetion de la lumiere, que chaque petit endroit d'un corps lumineux , comme le Soleil , une chandelle, Oll un charbon ardent, engendre fes ondes, dont cet endroit eil le centre. Ainii dans la flame d'une chandelle , eftalls diftinguez les points A,B, Cj les cercles concentriques, decrits amour dc chacun de ces points, reprefentent Ies ondes qui en provienent. Et il en faut concevoir de mefme amour de chaque point de la furface, & d'une partie du dedans de cette flame. Figure 4.7. Huygens' theory of light (1690).

64 A

F

Figure 4.8. 'Huygens' principIes (1690). Wavefront DCEF comes from A, but secondary waves such as KCL, arising from point B, also contribute to the effect.

with light and color (like that produced by Boyle) did not interest hirn. He ignored the colors, as they did not appear in his calculations. Huygens knew of Grimaldi 's work, but did not possess the mathematical instruments needed to explain the bending of light.

The speed of light For Aristotle, light was an instantaneous change in the nature of a medium. With few exceptions (Alhazen and Roger Bacon, for example) scholars upheld the idea of immediate propagation of light until weil into the 17th century. Even when ideas about light changed, some, like Kepler and Descartes, still maintained this belief. In Descartes' mechanistic universe, completely filled with inelastic partic\es, light was a mechanical disturbance, the impulse of which was propagated immediately, rather like someone with a long stick who can knock something over without anything moving between the hand holding the stick and the object. But once the peripatetic concept of the world was left behind, it became more natural to think of light as being propagated at a finite speed. Galileo was the first to devise an experiment to prove whether light needs time for its propagation (Fig. 4.9). Two people, each holding alantern, produce light flashes in turn by moving their hands which are screening the light; they do that every time they see a flash from the other lantern. Such a 'relay-

65

Figure 4.9. Galileo's plan for measuring the speed of light. The exchange of light signals between the two men should take longer as the distance between them increases (after van Heel & Velzel, 1967).

race' of light rays must, if light needs time to move, take longer when the men are further away from each other. The result was negative. Galileo conduded that the passage of light must take pi ace immediately or almost immediately. It was not until two hundred year later (in 1849) that the French physicist Armand-Hippolyte-Louis Fizeau managed to measure the speed of light in the laboratory. The distance light traverses in one second is astronomically great. Oie R0mer, who was the first to measure the speed of light, was in fact a Danish astronomer. In 1676 he calculated the speed of light from an apparent irregularity in the revolution time of the satellites of Jupiter. The irregularity could easily be explained by the fact that the light from Jupiter and its satellites reached the earth sooner when the two planets were doser together and later when they were further apart. R0mer made a reasonable estimate of the speed of light (wh ich is 300,000 km/sec). This velocity is so great that Galileo's experiment had to fail.

The refraction of light

The Greeks knew the laws of reftection (katoptrica) but not of refraction (dioptrica). In spite of Ptolemy's measurements they did not discover the law

66 according to which a ray of light is refracted when it passes from air to a denser medium, such as water. Alhazen had a theory of refraction but did not formulate it mathematically. He thought that during the transition to a denser medium the horizontal component of the incident ray was impeded (p. 26). Grosseteste thought that when light is refracted in water, the angle of refraction was half the angle of incidence (p. 30). Kepler stated in his Dioptrice that there was a fixed relationship between the angle of the incident ray and the angle of the refracted ray. This was then the best mathematically formulated law of refraction, but it was found only to apply to small angles of incidence. Snellius (Willibrord Snell van Royen, 1580-1626), a mathematician in Leyden, is usually credited with the discovery ofthe law ofrefraction [14]. (It has recently become known [15], however, that Thomas Harriott, an English mathematician who lived from 1560 to 1621, had made the discovery earlier.) In 1620 Snellius examined the relationship between the angles of incidence and refraction and found (Fig. 4.10) a simple mathematical relation: the relation between RV and RI was constant for all angles of incidence. This may also be expressed as: the relation between the sine of the incident ray and the sine of the refracted ray is constant. The discovery of the law of refraction was no mean achievement: a discovery made after a thousand-year search, and furthermore one of the first quantitative natural laws to be discovered.

L

/"

o

Figure 4.10. Snellius' sine law. For every angle of incidence i there is a constant ratio between RI and RY. It follows that the ratio between si ne and sine r is constant.

67 A

c Figure 4.11. Refraction according to Ferrnat. In the case of refraction into a denser medium, which slows down the light, the path ABC, dictated by the sine law, is the quiekest.

Descartes (1637) sought the origin of the si ne law and came up with an explanation. Light impulses (wh ich he had earlier said were propagated at infinite speed) move more easily through eompaet than through loose material. But this only applies to the component that is perpendicular to the refraeting surface. It is as if a tennis ball is hit with a racket in the direction of the refracting surface as it enters a denser medium. The inerease of velocity is (in Fig. 4.10) the relation Ol/OY. Descartes' theory is thus the reverse of AIhazen's: Alhazen says that the horizontal component is retarded by the denser medium, while Deseartes thinks that the vertical component is aeeeierated. Pierre de Fermat (1608-1665), a French mathematician, believed in a finite speed of light, and thought that acceleration of light in a denser medium was absurd. He adopted the opposite assumption and produced a new explanation (Fig. 4.11): when the image of point A, after refraction in a denser medium, is seen at point C, the path of light ABC dictated by the sine law is the quiekest. He based his argument on the Aristotelian law of eeonomy, the lex parsimoniae, a teleological law which did not fit very weIl into a mechanistic view of nature. Christiaan Huygens gave a better explanation on the basis of his wave theory of light. How the refraction of a wave front takes place when it enters a denser medium is made clear in Figure 4.12. Using this principle and the hypothesis that light travels more slowly in a denser medium, Huygens had no difficulty in explaining the sine law.

68

E

1'AB. 11.

F

Figure 4.12. Refraction according to Huygens (1690). The wavefront AC ofthe incident ray DA forms secondary waves at the surface of the water. These waves are retarded and form a new wavefront NB.

The rainbow The rainbow theory of Theodoric of Freiburg, the triumph of scholastic opties, had been forgotten two centuries later [16] (the manuscript was not rediscovered until 1814). In the Renaissance quite a lot had been written about the rainbow, but because the correct theory had been lost, the search had to start all over again. It took a long time for understanding of the rainbow to reach a point where it approached Theodoric of Freiburg's insight. The seventeenth century was the century in which the rainbow had to finally reveal its secret to Newton. Marco Antonio de Dominis made an important first step, Descartes elucidated the geometry of the rainbow but could not explain the colors; the color theory of Marci of Kronland was close to Newtons. Marco Antonio de Dominis (1564-1624 ) was a Dalmatian Jesuit who became archbishop of Spalato. After attempts to reconcile the Catholie creed with English Protestantism he went to Rome, hoping to become a cardinal. There he fell into the hands of the Inquisition and was killed. Around 1590 he made a study of the rainbow; an account of this work was published in 1611 as De radiis visus et lucis in vitris perspectivis et iride tractatus. Like Theodoric of Freiburg and Kamal al-Din he used a ball of glass as model of a raindrop, but his representation of the passage of rays was incorrect. He used

69 internal reftection (Maurolycus) and refraction (Delle Porta) in his theory, but he neglected the refraction of the ray when it leaves the globe. His idea that each raindrop only produced one color is correct, but he could not explain how that came about. His view of colors is partly peripatetic and partly modem. His explicit statement that apparent color consists solely of light itself is modem. On the other hand his view that all colors are formed out of light and darkness is Aristotelian. For example, the light of a fire becomes red because it is darkened by smoke; dark mountains in the background cause light to lose all its colors except blue. Green occupies an intermediate position between red and blue. Rene Descartes is the man who rediscovers Theodoric's theory (Fig. 4.13). He also makes an important addition: the explanation of why the rainbow has a radius of 42°. On the basis of the law of refraction he calculates that, in the raindrops of the main rainbow, with two refractions and one reftection, the majority of rays must emerge in such a way that they are seen at an angle of 41 ° to 42°. For the secondary rainbow these values are 51 ° to 52°. Descartes . '.

~

. ","

A

.. .

t···· .. ···· ........ ·

-

z

.'

'-,

...,:. ".:.""'. . '

--- . -" . .-=-

.

---'"'--::-;;

Figure 4.13. Descartes' theory of the rainbow (1637). In the main rainbow light undergoes two refractions and one reftection. The secondary rainbow (dotted line) is the result of two refractions and two reftections.

70

Plate 1. A double rainbow above Newton's place of birth (Photograph taken by R.L. Bishop, Acadia University, Canada).

was also able to explain 'Alexander's dark zone', outside the main rainbow (see Plate 1). This zone had been described by Alexander of Aphrodiasis, who was head of the Lyceum from 198 to 211. Descartes was not able to explain the colors satisfactorily, although he hirnself considered that they followed automatically from the rotatory movements of small balls of light. Johannes Marcus Marci of Kronland (1595-1667) was a Jesuit who had studied medicine and became the personal physician of Kaiser Ferdinand 11. He was an all-round scholar who wrote about everything, including the way to make gold. He is called the 'Bohemian Galileo', but his publications did not easily reach the West. Central Europe had become isolated by the Thirty Years' War and natural science moved (from Italy and elsewhere) to France,

71 the Dutch Republic and England. Thus Descartes' work had not reached Marci. In 1648 Marci wrote a voluminous book on the rainbow: Thaumantias, fiber de areu eoelesti. To increase his knowledge of colors he studied the passage of light through a prism (which he called an iris trigonia). Although his light theory was faulty (he distinguished 'colorigenic' and 'photogenic' rays in light), in some respects he anticipated Newton, as evidenced by the following remarkable observations (theorem XX): 1. It is not possible either to have the same color with a different refraction, or different colors with the same refraction. 2. A colored ray which undergoes a further refraction does not change color [17]. Here, for the first time, in an almost forgotten book which had no consequences for the evolution of color knowledge, a constant relationship between color and refraction is assumed, and even Newton's experimentum erucis is preconceived. But Marci thought that white light was homogeneous and was only modified by a prism. Iris, the messager of the gods, had to wait another twenty years before she was finally unveiled by her fairy prince: Isaac Newton.

The chemical colors Important advances had been made in the theory of color in the seventeenth century, but they had mainly been made in the field of the 'false' colors, knowledge of the 'true' colors had stagnated. I shall adopt Goethe's terminology and call false colors (which can be understood with knowledge of classical physics) 'physical' colors and true colors 'chemical' colors. The problem of the chemical colors was at that time insoluble; it was not until the twentieth century that there was any understanding of the connection between the color of a substance and the structure of its molecules. In the seventeenth century there were only two ways of considering chemical colors: acknowledging their existence and resorting to speculative alchemistic ideas to explain them, or denying their existence as aseparate entity and converting them into physical colors. Isaac Vossius (see p. 75) tried the first solution, Robert Boyle the second. Robert Boyle (1627-1691), son of an English nobleman, travels through Europe in his youth and is attracted by the mechanistic and corpuscular line of thought. Later he practices experimental physics and chemistry. He rejects (in the Seeptieal Chymist 1661) the peripatetic doctrine of the four elements and Paracelsus' theory of the three Principles. He gives a new definition of the word 'element' on the grounds of Gassend's corpuscular theory.

72

In 1664 he publishes Experiments and Considerations touching C%urs, a book of more than 400 pages, the first large English contribution to the problem of color. In Baconian tradition he gives a detaiIed natural history of the colors. He calls red, yellow and bIue, more explicitly than Aguilonius and Grimaldi, the primary colors and thus anticipates the trichromatic system (1664): There are but few simple, and primary colours; from the various compositions whereof, all the rest result: for tho' painters imitate the hues of those numerous different colours, to be met with in the works of nature and of art; yet I have not found, that to exhibit this strange variety, they need employ any more than white, black, red, blue and yellow; these five, variously compounded, and re-compounded, being sufficient to exhibit such a variety, as those, who are altogether strangers to the painters pallet, can hardly imagine [18]. Boyle disputes the separate identity of the chemical colors and declares that these are also formed by modification oflight. He is interested in the color of indicators like litmus; he discovers that the color change to red is brought about by all acids and to blue by all alkalis. lust as unreal and temporary is the color of lignum nephriticum, which he, like Kircher (p. 74), describes in detail: looked at from above, the liquid is blue, but when you look through it it is yellow. But solid substances are also continuously modifying light: for instance, gold foil seen in reflected light is yellow but it is blue-green when looked through. Colored objects, even those that are smooth, are seen through the microscope to have a rough surface with which light can be modified. His explanation of white and black is interesting and accompanied by original experiments. White is the result of complete reflection, presumably from round, convex particles of matter. Black arises from complete absorption; for this reason a burning-glass scorches a black cloth more quickly than a white one. Black particles of matter, which capture the light, are naturally hollow, concave. Other colors cannot be produced by reflection and absorption, but by refraction and other processes which give rise to 'emphatic colour'. Boyle refers in this connection to the colors in a soap bubble. For the further study of the phenomenon of refraction Boyle allows light to fall onto a prism in a darkened room (Fig. 4.14). The triangular prismatic glass being the instrument upon whose effects we may the most commodiously speculate the nature of emphatical colours (and perhaps of the others too) [19]. Two reflections occur, two main rainbows caused by refraction alone and two secondary rainbows caused by refraction and two internal reflections. He distinguishes five colors in the rainbow: red, yellow, green, blue and purple.

73

A - - -- --

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.... -

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...

(C.

I I I

I

I

4

I

s

I I

I

I

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I

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,.-

'I-

, I

I

/.,.. Figure 4.14. Prismatic color dispersion (Boyle, 1664).5,4,3,2,1: purpie, blue, green, yellow, red. There are four spectra. AE is refracted, Al is also reflected twice.

Boyle does not know which modification of light produces color: change in the speed of the light particles, in their size or in their rotation. Finally Boyle poses another difficult question: why don't the light particles prick the retina like a needle, instead of bringing about the sensation of color? In this connection Boyle uses for the first time the term 'secondary quality' for a sensory experience. In accordance with mechanistic philosophy, 'primary' is for Boyle solely the mechanical interaction between particles of matter.

The color theories of opponents of the corpuscular hypothesis

Even the 'revolutionary' seventeenth century has, as is to be expected, its conservative Aristotelians. Aguilonius has already been mentioned in this connection but, as 'perspectivist' and thus follower of Alhazen, he was more a

74 conservative from an earlier century. Typical representatives of the peripatetic color theory in the seventeenth century are Athanasius Kircher and Honore Faber. Other conservative color theorists think more along Renaissance lines. Isaac Vossius bases his ideas on the Egyptian-Hellenistic sulfur-mercury theory of the alchemists (modified by Paracelsus to the theory that all substances are built up of the three principles Mercury, Sulfur and Salt), and also on the opinions, inftuenced by pre-Socratic thought, of Bernardino Telesio. And finally, in the time of the scientific revolution, neoplatonism and the light metaphysics of the Renaissance still have an advocate: Marin Cureau de la Chambre. Although these four color theorists were in agreement as far as their opposition to mechanicism was concerned, they differed greatly in other respects. That is especially true for their ideas about true and apparent colors. For Athanasius Kircher and Honore Faber there is absolutely no reason to alter the peripatetic definitions. Vossius, on the other hand, says that the apparent colors are true colors. De la Chambre declares in an extremely pretentious light theory that apparent and true colors exist but that they are both equally real. Athanasius Kircher (1601-1680), a South-German Jesuit, is a colorful know-all who teaches mathematics and Eastern languages in Rome. He is known for his invention of the magic lantern. In 1646 he publishes a great book Ars magna lucis et umbrae in Rome. He adopts the Aristotelian standpoint - as apparent from the title - that color arises from a combination of light and shadow. The rainbow is caused by prismatic action: the rays which take the shortest route through the prism are red, a longer passage through the prism darkens them to blue. Even the clearest glass darkens light. It is Della Porta's theory over again. Kirchner's linear color scheme resembles closely that of his kindred spirit, Aguilonius. Kircher's scheme is more detailed, without becoming clearer (Fig. 4.15). Kircher mentions two optic phenomema which interested people at that time: the stone of Bologna and nephritic wood. The Bolognese phosphorus is a sulfur-holding mineral found near Bologna which, after calcination, becomes phosphorescent and glows in the dark. The phenomenon that a visual image continues to be luminous for a moment after the light is out, is compared by Kircher with the afterglow of Bologna stone after it has been exposed to sunlight. It is a curious comparison between two phenomena which have little in common. Halbertsma (1949) seems therefore to be stretching a point when he says that Kireher was 'the first physiologie al colour expert' . Nephritic wood is a South-Ameriean sort of wood from whieh an infusion was made for the treatment of renal colic. When looked at from above the

75

Figure 4.15. Kircher's color scheme (Ars magna, 1646). The sequence is the same as in Aguilonius' scheme (Fig. 3.1), but there are more composite colors.

liquid is blue, and when looked through it is yellow. Nephritic wood is a favorite introductory theme to a treatise on true and apparent colors; in that connection it was also mentioned in the section on Boyle. Honoratus Fabri SJ. (1607-1688), professor of mathematics and philosophy in Lyon, is a kindred spirit of Kircher. He writes a treatise on color in 1670, De coloribus. He gives a quantitative specification of the colors, in the way that Aristotle had proposed (p. 11). The proportion of light to darkness in red is one to one, in yellow two to one, in blue one to two. One hundred and fifty years later a similar hypothesis was to be defended by Schopenhauer in Germany (1816). Isaac Vossius (1618-1689) was a comprehensive Dutch scholar. He was interested in history, classical languages and Arabic, mathematics, physics and geography. He was known to be conservative and was a fervent antiCartesian. Although he was a protestant he published St. Ignatius Loyola's letters. He published De lucis natura et proprietate in Amsterdam in 1662. According to Vossius warmth is the source of all color. That is the reason that all animals in the Polar regions are white. One tropical bird has more color than all the birds together which live in the northem forests. The basis of color is sulfur, which can assume all colors under different circumstances: from white via yellowish-green to flaming red, then to blue and even, at the end of the chemical process, black. It is incorrect to describe color as modified light: colors and light have nothing in common. All light which comes from fire - and that includes the sun - contains sulfur. Apparent colors are really true colors which are carried by light; they can be seen by passing sunlight through a prism. With this last idea Vossius anticipates Newton, although his

76 color theory is mainly influenced by the theories of Bernardino Telesio and Paracelsus. Marin Cureau de la Chambre (1594-1667) is the personal physician of the French kings Louis XIII and XlV. In his book La [urniere (1657), dedicated to cardinal Mazarin, he attacks Aristotle's light and color theory. Light cannot mix with darkness: no one has ever seen that light became colored by being weakened. The idea that color dispersion in a prism occurs because blue light follows a longer trajectory through the glass than red is ungrounded; a prism made of dark glass produces the same color dispersion as anormal prism. Vossius' theory cannot be true either. If color is associated with the flaming phi10sophic princip1e Sulfur, a ruby would lose its color in water, because water is the enemy of flame. And Boyle's corpuscular theory cannot be true either: when ground white and black marble are studied under the microscope, all the particles look exactly the same. De la Chambre places light and colors in a neoplatonic setting. Light, the most sublime and noble of all perceptible qualities, is exalted above the lowest and most imperfect thing that exists: matter, and flees from it with distaste. Light is immaterial, non-substantial. For this reason, according to de la Chambre, the bread remains white after the Transsubstantiation; it cannot change color because color is not a substance [20]. (Thus, theology may contribute to science!) 'True' colors are luminous colors inside matter. It is understandable that matter contains inner light because light generated space and dragged matter along with it (see quotation from Grosseteste, p. 28). This inner light is so weak that it cannot be seen in the dark, it only becomes visible when it combines with extern al light (the exception is the Bolognese phosphorus). In the rainbow it is the external light itself which adopts the color of a cloud. Thus true and apparent colors do not essentially differ and are both equally real.

v Newton Newton marks the beginning of a new era in physics. The various lines of thought described in the last chapter converge towards his philosophy of nature: Copernicus' heliocentric system, Kepler's study of optics and the movement of the planets, Galileo' s theory of the free fall, the mechanicism of Descartes and Huygens, and the atomism of Gassend. Because Newton is unique in the contribution he made to the theory of color, a few biographical notes are given here [1]. Isaac Newton (1642-1727) was born in the year of Galileo's death. The complete 'scientific revolution', in which they played such an important part, occurred within the life span of these two men. Newton comes from a family of gentlemen farmers. Being uncommunicative and unsuited to a farmer's life, he is sent to Trinity College, Cambridge. During the years 1664 to 1666 Newton hirnself is almost the only witness of the first expression of his genius. He studies Descartes, Galileo and Kepler. He is particularly interested in mathematics and mechanics and investigates the chromatic aberration of telescope lenses. At feverish speed he writes one treatise after another, but he does not publish anything. In 1672 he finally makes his first public appearance with a letter to the Royal Society on light and color. He immediately regrets that he has emerged from anonymity. The criticism of scholars like Hooke and Huygens makes hirn decide not to publish anything again. For many years he lives in isolation in Cambridge. In the hope of gaining further insight into the secrets of nature and of God, he adds intensive study of alchemy and theology to his investigations in physics and mathematics. Newton's life undergoes a radical change when the astronom er Halley hears by chance that Newton is able to explain the orbits of the planets by means of his own discovery: calculus. Halley insists on publication. Within two years Newton has written his Philosophiae naturalis principia mathematica, printed at Halley's own expense (1687). The book contains a new system of mechanical axioms, including the three laws which are named after hirn. It also provides proof that the planets must move according to Kepler's laws if they are inftuenced by apower which is directed towards the sun. Finally, Newton identifies this power, on the basis of calculation of the movement of the moon, as the power of gravity.

78 A few years after completing the Principia, the book that made hirn famous, Newton has a nervous breakdown which terminates most of his scientific creativity. He decides to go to London, where he soon becomes the Master of the Mint and also Chairman of the Royal Society. At this time he writes his other important work, the Opticks (1704), the description of investigations which he hadperformed thirty years earlier. The book, which is much easier reading fornon-mathematicians than the Principia, had great infiuence. It dominated with alm ost tyrannical authority the eighteenth-century ideas of optics and colors. It is understandable that Newton encountered much criticism in his lifetime. The French mechanistic school recognized nothing but matter and movement. A gravitational power working at a distance was treason and regression to medieval thought. Newton defended hirns elf with this pronouncement: I feign no hypotheses. And to us it is enough that gravity really exist and act according to the .laws which we have explained [2]. By accepting the adequacy of mathematical relationships and avoiding ungrounded hypotheses, Newton set an example to the many physicists who were to come after hirn. Incidentally, his rejection of hypotheses also had an apologetical character. His firm belief in God, omnipresent and everlasting, forbade hirn to inquire into the fimil secrets and strengthened his distaste for Descartes' mechanicism. Nevertheless, the Principia, more than any other book, is responsible for the final disenchantment of the world. A new theory of light and color

Lenses and the lens combinations of telescopes brought Newton into contact with colors. Whenexamined through telescopes, objects had colored contours (now called 'chromatic aberration', seeFig. 5.1), which limited the usefulness of the lenses. Newton wondered if the chromatic aberration could be prevented by grinding the lenses aspherically. For further study he acquired glass prisms; it was common knowledge that prisms (and water-filled glass globes) could produce colors; Seneca had already described tbis phenomenon. In the earlier chapters of tbis book the rainbow colors produced by prisms were rnentioned in connection with Grosseteste, Alhazen, Theodoric, Descartes, Marci and Boyle. Some of these had performed careful experiments, for example Witelo and Boyle, who let light fall through a small hole onto a prism in a dark roorn. Newton did the same, but with greater experimental talent. He noted that a prism only dispersed the sun's rays in one direction, the direction perpendicular to the ribs of the prism. Thus the plism rnakes a long colored stripe frorn a mund spot of light. Red is refracted the least and violet the most, in between lie the colors orange, yellow, green and blue.

79

w

R

Figure 5.1. Chromatic aberration. The violet rays from W have a shorter focallength than the red rays.

Experiments with a second prism were of crucial importance. It appeared that part of the spectrum, for instance the red, could not be further dispersed by a second prism. Newton called this the experimentum erucis (Fig. 5.2) and on this basis built up his hypothesis that white light is composed of spectral rays which are refracted by a prism to varyingdegrees. What Newton found most extraordinary was the fact that the prismatic colors could be recombined to white if they were allowed to pass through another prism placed in the right position (Fig. 5.3). White is only a combination ofprimary (spectral) colors! Newton's conc1usion was self-evident: color is a primary property of light itself. It is not a property of objects, nor a 'modification' of light through a prism. Objects are colored because they reflect more of one portion of white

5

E Figure 5.2. Newton's experimentum crucis. A spectral color g cannot be dispersed by a second prism (Opticks, 1704).

80 L

Figure 5.3. White light is composed of colors. Light incident at F fonns a spectrum p-t. The rays are re-uni ted by a convex lens to form white at x, and at y are refracted for the second time.

light than of another. And of course, the rainbow was no problem for Newton any more either. As the spectral colors were apparently separate 'primary' (we should say: monochromatic) sorts of light, Newton concluded that chromatic aberration through lenses was unavoidable and telescopes with mirrors instead of lenses would have to be built. Newton constructed a prototype of such a reftecting telescope and sent it, together with a paper on his new theory, to Oldenburg, the secretary of the Royal Society (1671). The New Theory of Light and Colour was published in the Philosophical Transactions of the Royal Society of London. From Newton's covering letter it is clear that the letter contains more than a mere report on experiments, but in fact a new hypothesis: a letter of Mr. Isaac Newton, Mathematick Professor in the University of Cambridge; containing his New Theory of Light and Colours: Where Light is declared to be not Similar or Homogeneal, but consisting of difform rays, some of which are more refrangible than others: and Colours are affirm'd to be not Qualifications of Light, deriv'd from refractions of natural bodies (as 'tis generally believed); but Original and Connate Properties, which in divers rays are divers: Where several Observations and Experiments are alleged to prove the said theory. (Newton, 1672). Newton's experiments were simple, weIl thought out, and for most people convincing. But there were still a few critics. Hooke and Huygens doubted the evidential value of his experimentum crucis. According to them, only the connection between color and refrangibility had been demonstrated. Why should a second prism have the same effect on a colored beam as the first had on the white beam? The old idea that a prism produces colors by modifying the light did not seem to be refuted. (Their objections would have had even more weight if they had known that what happens when white light passes

81

through a prism is not only dependent on the refractive power of the prism, but also on the color-dispersing power of the glass). In fact, Newton's argument was extremely powerful, but mainly within the limits of his own hypothesis of light. Disloyal to his own rule that he did not invent hypotheses, Newton, like Boyle, was really a supporter of the corpuscular theory of light. This is clearly, although not explicitly, expressed in the opening sentence of the Opticks: By the Rays of light I understand its least Parts, and those as weIl Successive in the same Lines as Contemporary in several Lines [3].

In other words, a ray of light is not a straight line of light, but only a light particle on that straight line. (We should now - with much reservation - call it a 'light quantum'). Strangely enough, Newton continues to use the word 'ray', even when he means the various sizes and configurations of the light particles. Following in Gassend's footsteps, Newton's point of view is that atoms are unalterable - light atoms are therefore diverse from the beginning, and are not modifiable by prisms. Qu. 27: Are not all Hypotheses erroneous which have hitherto been invented for explaining the Phaenomena of Light, by new Modifications of Rays? For those Phaenomena depend not upon new Modifications, as has been supposed, but upon the original and unchangeable Properties of Rays [4]. Newton's most powerful argument for the corpuscular nature of light was the linear propagation, which is difficult to reconcile with the hypothesis of waves. As was to be expected from the founder of classical dynamics, Newton regarded reftection, refraction and diffraction as the products of repelling and attracting powers. The difference in refraction of the spectral colors is due, according to Newton, to the fact that violet light particles are much smaller than red ones and diverge much further from their original path through the attracting power of a denser medium, in which the speed of light, according to hirn (and Descartes and Hooke), is greater than in air. Newton also easily explains the sine law on the grounds of the dynamics of light particles. Newton studied colors in thin layers as can be seen in a soap-bubble. His riyal, Hooke, had described them already and considered their existence to be his experimentum erucis, proving that the origin of colors should not be sought in prismatic action. Newton pressed a ftat glass against a convex glas of known curvature and saw concentric rainbows, 'Newton's rings'. There were alternating zones of reftection and transmission (Fig. 5.4) on the concave surface. He discovered that rings of the same color always occurred where the thickness of the layer of air was a multiple of a given minimum thickness (Fig. 5.5).

82

Figure 5.4. Newton's rings. When a convex glass is pressed onto a plane gl ass, concentric colored rings are formed of alternate reflection and transmission.

Figure 5.5. The position of the colors in Newton' rings. The points on AB where red is reflected are located where the distances between the plane and the convex surface are a multiple of the shortest distance.

The ether, which had been indispensable to Huygens, could not be completely thrown overboard by Newton. The problem of the partial reftection of light, as from a glass plate, is resolved with the help of that same ether. The light particles on a half-reftecting and half-transmitting surface give rise to vibrations in the ether which, depending on their density, either cause 'fits of easy transmission' or 'fits of easy reftection'. According to Newton, the rings in thin layers are also associated with these 'fits': differently colored light rays cause the ether to vibrate at different frequencies, as the result of which the light, at different layer thicknesses, is altemately held back or allowed through. Ether is an even finer substance than the light particles, and slows them down a little. Although Newton's preference for a corpuscular theory is evident, he is very circumspect on this point, partly in order not to exacerbate his disagreement with Hooke.

83 'tis true, that from my theory I argue the Corporeity of light; but I do it without any positiveness [5]. Making too sharp a distinction between Huygens' wave hypothesis and Newton's corpuscular hypothesis does not do justice to the ambivalent positions of the two scholars. Huygens presupposed particles as the substratum of his 'waves', Newton used the hypothesis of vibrations in the ether as conductors of his light particles. It was not until after Newton's death that the corpuscular theory of light became a dogma, at least in England, in the eighteenth century. In actual fact, neither Newton nor Huygens had been able to devise a theory which could explain a11 the then known characteristics of light. It took another century before a new theory of light was conceived, which could explain both the colors in thin layers and the rectilinear propagation of light. It was a theory which elaborated on Newton's principle of the heterogeneity of white light and on Huygens' idea of secondary light waves.

Newton's color system Newton's discovery that the spectral colors are the components of the 'color' white is one of the great discoveries of physics. Newton is justified in ca11ing it (in his letter of 1672 to the Royal Society) the most considered detection which has hitherto been made in the operations of nature. But it is a purely physical discovery, in which the word 'color' is only used figuratively. Being an adherent of the mechanistic theory of light, Newton realized (like Galileo) that colors are -in Boyle's nomenclature - 'secondary qualities' . And if at any time I speak of Light and Rays as coloured or endued with Colours, I would be understood to speak not philosophica11y and properly, but grossly, and according to such Conceptions as vulgar People in seeing a11 these Experiments would be apt to frame. For the Rays to speak properly are not coloured. In them there is nothing else than a certain Power and Disposition to stir up aSensation ofthis or that Colour [6]. This distinction between primary and secondary qualities ca11s for a new doctrine: the theory of the relationship between physical color stimuli and mental color sensations. This williater become one of the main themes in the science of color, but Newton sketches the new field in less than 40 pages with shrewd intuition. His experiences with the mixing of monochromatic rays fina11y induce Newton to devise a system of color vision. He mixes spectral rays in two different ways.

84 In the first method a sort of comb is used to select parts of the spectrum - and intercept other parts - before the spectrum is recombined by means of a lens and a second prism. Using this method he makes a fundamental discovery: by mixing red and yellow rays in the right proportion he obtains an orange color which is indistinguishable from monochromatic orange. It appears thus that totally different rays are able to produce two identical colors (such colors are now called metamerie colors). He also discovers that mixing the two ends of the spectrum produces a color that did not occur in the spectrum: extraspectral purple. In the second method Newton obtains with two prisms two spectra which are so composed that the violet of the first spectrum covers the yellowish green of the second spectrum, the indigo of the first covers the yellow of the second and the blue-green of the first covers the orange of the second. According to Newton all these combinations of two spectral colors produce white. It is not pure white, and Newton remains uncertain whether it is possible to obtain pure white from a mixture of two colors; he only obtained pale 'anonymous' colors. But he has no doubts of being able to obtain pure white from a mixture of four or five spectral rays. Without any further introduction he then presents his color circle (Fig. 5.6). The spectrum is curved into a circle, so that red lies next to violet, and white is placed in the middle. In the spectrum seven colors are distinguished which gradually merge into each other. In this way Newton introduces for the first time a practical, psychologically justified, system of colors. The circumference of the circle represents the saturated colors (although he forgets extraspectral purple). White is in

~""""----k\

Figure 5.6. Newton's color circle (1704). The spectrum has been bent so that violet and red meet. White is at the center.

85

the middle and all the unsaturated colors fill the plane between white and the circumference. The unsaturated colors contain white and have the same tint as the spectral color which lies on the extension of the line between the unsaturated color and white. This psychological arrangement of colors in a plane is perhaps the most important discovery in the whole science of colors. Until then, Aristotle's linear color scheme had always been accepted, an arrangement on one line between white and black. Newton realizes that the colors are arranged in a circle; although he forgets purple it will not have escaped his notice that there is a subjective affinity between red and violet. In addition he assigns a place to all the unsaturated colors. With his color plane he added a dimension to the linear color scheme of Aristotle and of all other color theoreticians, such as Aguilonius, whose color arrangement is depicted on page 38.

The barycentric system Newton's color circle is not only a psychological, but also a psycho-physical scheme. Newton indicates - again with no introduction - how colors can be obtained by mixing spectral rays. Given that the spectral colors p, rand t (spectral red, yellow and green) are mixed in given proportions, the resulting color can be the color z, a pale orange. If we know the number of spectral red, yellow and blue rays, then z is the common center of gravity of p, rand t. This procedure is clarified for the modern reader in Figure 5.7. With this barycentric system Newton lays the basis for colorimetry, the quantitative theory of the connection between mixing of rays and the resultant color. Newton hirns elf calls it a practical approximation without mathematical precision. Of course that is true, because how could he quantify the 'number of rays' in 1704? That point in color science was not reached until a century and a half later. But Newton's color plane and his barycentric system of color mixing have remained the basis of colorimetry. Figure 5.7 shows us that Newton's collection of colors is implicitly threedimensional. There is a plane on which all the hues and all the degrees of saturation are represented and there is the direction of brightness perpendicular to it. Newton hirnself did not realize this; he was too much occupied with the analogy between colors and tones. On the grounds of his quantitative experiments with 'Newton's rings' he had come to the conclusion that the range of colors from red to violet spanned an octave of vibrations in the ether. For this reason he divided the circle into seven colors, in intervals which corresponded with the seven tones of the Dorian scale [7]. As stated in Chapter 1, Aristotle had already compared the colors to the tones and tried to find a numerical system for colors. Others had followed Aristotle in this concept. For Newton,

86

Figure 5.7. Explanation of Newton's barycentric system. Color A has more weight than color C. When these calors are mixed the weight of the resultant color inserts at the point of gravity between A and C.

respect for God's creation was the reason to look for an analogy between light and music: Whence is it that Nature does nothing in vain; and whence arises all that Order and Beauty which we see in the world? Was the Eye contrived without Skill in Opticks, and the Ear without knowledge of Sounds? [8]. The physiology of color vision

Newton not only makes statements about the physics and psychophysics of colors, but also about the physiology. Light rays cause 'vibrations of several bignesses' in the retina, which give rise to various color sensations, just as vibrations of air produce different tones. These vibrations are conducted along the nerves to the brain. Like Aristotle he considers that the harmony of colors is based on the relationship between the frequencies of the vibrations, in the same way as the harmony of tones. He is struck, en passant, by a particularly daring thought about the path of the visual nerves to the brain. In order to explain binocular vision, he assumes that the fibers from the temporal half of the left retina and the nasal half of the right retina combine in the chiasma so that double vision is prevented and the right visual field is represented in the left half of the brain [9]. The 1eft half

87

.... .'.~~ ,'rl .'

Figure 5.8. Newton's hypothesis of half crossing of the optic fibres in the chiasma. Thus fibers from 'corresponding retina! points' reach the same cerebra! hemisphere. (manuscript, from Grusser, 1990).

of the visual field is represented in the same way in the right half of the brain (Fig. 5.8, [10]). This idea that only half ofthe optic fibers cross in the chiasma was adopted by the medical world, but was only confirmed anatomically at the end of the previous century.

VI From Newton to Young Newton's color theory, published in the Philosophical Transactions of 1672, forced physicists to make their position clear [1]. They had to make choices with regard to two separate questions. The first question was: is white light heterogeneous, consisting of rays which are refracted differently according to their color? Or is white light homogeneous, but able to be modified by a prism to form a colored spectrum? The answer to this question depended on how Newton's experiments were viewed, especially the value attached to the experimentum crucis. Newton himself chose unequivocally for the first option, the heterogeneity of white light. But the natural philosophers also had to make a choice regarding another theoretical problem: does light consist of corpuscles, racing at incredible speed through empty space, or is light an impulse, a vibration, which is propagated through an ubiquitous medium? The Cartesians preferred the medium hypothesis, while the Gassendists chose for the corpuscular nature of light. Newton chose the corpuscular hypothesis, although not without some hesitation. But he was adamant in his opinion that the nature of light was mechanical; he rejected categorically the 'notion of the Peripateticks' that colors were qualities and that light was an accidens. What were the qualities then, if they were not properties of light? This question was tackled by scholars - led by Lomonosov - in the second half of the eighteenth century. That was the first time that seeing colors became a topic of discussion. This chapter culminates in the figure of Thomas Young, one of the initiators of the wave theory of light, and also the true founder of the physiology of color vision. After Young, this history of the theory of color becomes the history of the theory of color vision. After the appearance of the Opticks in 1704, and after a few problems in Newton's publication of 1672 had been cleared up, it became practically impossible not to take up a position in relation to Newton. The only available options were to be a Newtonian (with the choice between the corpuscular or medium hypothesis) or an anti-Newtonian. The anti-Newtonians did not form a homogeneous group. It included physicists, like Hooke, Huygens and Mariotte, who were not convinced by Newton's experiments. But the majority of the anti-Newtonians did not primarily oppose Newton's optics but his mechanistic philosophy. Newton, the English

89 Protestant heretic, had become the symbol of the new secular ideology, in which science was taking the place of religion. The stumbling-block for the anti-Newtonians was, of course, the Principia, the book which contained explosive cosmological material. For the middle classes, however, without specialized knowledge, who were either in favor of or against the Aristotelian doctrine of the established ecclesiastical order, Newton's cosmology was too difficult. Thus, strangely enough, the color theory became the bone of contention between Newtonians and anti-Newtonians. But the Newtonians were not only battling with peripatetic conservatives; supporters of Descartes and Leibnitz were also represented. For example, Francesco Algarotti's much-read popular work: II Newtonianismo per le dame (1737), through which Newton's philosophy spread to the boudoirs and salons, was directed against the Cartesian philosophy [2]. Color theory was also of interest to another group in the eighteenth century world: people who were practically involved with colors, artists, printers and cartographers, who, each in his own way, had arrived at a color system without much theoretical backing. This group, which was interested in the arrangement of colors, could leam a lot from Newton's color circle and his barycentric model of color mixing: that short, but extremely important, part of Newton's Opticks. When trying to find one's way in the rather chaotic color theories of the eighteenth century, one is repeatedly confronted by three questions which will not receive an answer until the nineteenth century: 1. What is the relationship between color and sound? This question is of fundamental importance for physicists, and the correct ans wer, given by Young and Fresnel, led finally to a completely new theory of light and color. But there is a more tradition al kind of interest in the connection between color and music. It is an ancient theme, but deserves attention for more than its anecdotal character alone. 2. The second subject which receives increasing attention in the eighteenth century is that of the 'primary' colors. Is there an infinite number of primary colors? Or seven, or four, or three - or only two? Those who express an opinion on this subject do not always confine themselves to a phenomenological description of the colors, but sometimes couple a personal theory with the postulated number of primary colors. 3. The third question which keeps cropping up is: Is there a difference between the mixing of pigments and the optical mixing of colors?

90 The reception of Newton's color theory In his own country Newton's color theory was soon almost universally accepted: light consisted of the emission of corpuscles of varying refrangibility. The objections which some specialists, like Hooke and Huygens, had to this theory, were based on the double refraction of light and the colors in thin layers, difficult phenomena which did not interest the general public. William Molyneux (1656-1698), an Irish politician and author of the Diortrica nova (1692), was an early supporter ofNewton. He would probably have been forgotten by now if he had not, in a letter to John Locke, formulated the famous question: Would someone, who had been born blind, be able, after a successful operation, to distinguish a triangle from a square? This question has become known, in the history of visual theory, as Molyneux's Premise. Samuel Clarke (1675-1735), theologian, mathematician and physicist, was an important young supporter of Newton. They had much in common: both were mathematicians and physicists, and (secret) adherents of Arianism. Clarke, who translated the Opticks into Latin, did much to spread Newton's theories; he was, in fact, his spokesman. Clarke's letters have made him famous, especially his correspondence with Leibnitz on the subject of space. And finally, Jean Theophile Desaguliers (1683-1744), a Protestant theologian and refugee from France, who later became Professor of Physics at Oxford, was an extremely good experimenter and, at Newton's request, repeated meticulously many ofhis optic experiments. He also defended Newton's philosophy on the continent, in the Netherlands and elsewhere. According to Van Musschenbroek 'he combined conspicuous dexterity in the performance of experiments with supreme eloquence'. On the continent, Newton's theory was soon accepted in the Dutch Republic and Germany. Willem Jacob's Gravesande (1688-1742), originally a lawyer in The Hague, met Newton in 1715 and, on his recommendation, beca!lle Professor of Mathematics and Astronomy in Leiden. He wrote a N ewtonian textbook with the (shortened) title: lntroductio ad philosophiam Newtonianum (1720-1721), which was also much read in English and French translations. When Voltaire was writing his popular work Elemens de la philosophie de Newton (1738) he first attended lectures given by 's Gravesande in Leiden. Petrus van Musschenbroek (1692-1761) was the other Dutch propagator of the Newtonian philosophy. He had studied medicine with Boerhaave, but became Professor of Mathematics and Philosophy, successively in Duisburg, Utrecht and Leiden. He wrote aseries of treatises on the philosophy of Newton, whom he praised for his 'divine sagacity' he was also much read in Germany [3]. In Germany, Newton's color theory (at least his doctrine of the heterogeneity of white light) was at first warmly received, especially by

91 the mathematician Christian Wolff (1679-1754). Wolff [4] was also the most important German philosopher, but his philosophical viewpoint was quite different from that of Newton: he was a Leibnitzian. Newton's theory did not reach France untillater [5]. This was mainly due to Mariotte, who repeated Newton's experiments with insufficient accuracy. Edme Mariotte (1620-1684) was an authoritative French physicist, a founding member of the Academie Royale des Sciences (1666) and co-discoverer of 'Boyle's law'. He also discovered the blind spot in the visual field; on the basis of the fact that the choroid (the vascular membrane enveloping the retina) is absent at the spot where the optic nerve leaves the eye, he drew the incorrect conclusion that the choroid was the light-sensitive organ. In 1681 Mariotte wrote De La nature des couleurs. He had been impressed by Newton's 1672 publication and had repeated the experimentum cruds. When he allowed the violet light to pass through a second prism it was seen to contain red and yellow light. So Newton must have been wrong: the second prism 'modified' the violet light! Closer study of his spectrum (which had been produced in a slipshod experiment) showed that it was white in the middle, with blue on one side and red and yellow on the other. Green only appeared between yellow and blue when the screen was held at a greater distance from the prism. Green light was thus apparently a mixture of yellow and blue light, just as the color green is obtained by mixing yellow and blue paint. Mariotte concluded that there were three primary colors: red, yellow and blue. Was it really a conclusion? It seems more probable that an experiment could not deflect Mariotte from the traditional prejudice that green was not a primary color. It was not until 1715 when, at Newton's request, the experimentum cruds was repeated by Desaguiliers at the Royal Society in the presence of various French scholars, that people in France became more convinced of the accuracy of Newton's experiments. Nevertheless, the anti-Newtonians continued to invoke Mariotte in their defence. Buffon complains in 1743: Although people have much occupied themselves with the physics of color, little progress seems to have been made since Newton. It is not that the subject has been exhausted, but the majority of the physicists have more tried their best to combat Newton, than to understand hirn [6]

Supporters of the medium hypothesis Nicolas Malebranche (1638-1715), the author of Reflections sur la lumiere et les couleurs (1699) was theologian, philosopher, mathematician and physicist. He was an admirer of Descartes and tried to reconcile his doctrine with theology. Like Descartes, he considered that light was a disturbance of

92 equilibrium in aspace completely filIed with etheric material. Unlike Huygens, who assumed the existence of aperiodic impulses, he thought of light as a 'vibration of pressure' in the ether, and colors as vibrations of different frequencies. This thought had occurred to hirn by comparing light with sound. Later he adopted Newton's hypothesis that white light is heterogeneous. Like Newton he assumed that there are seven colors, arranged like a musical octave. Malebranche was the first to give a plausible explanation of colors within the framework of the medium hypothesis, but his theory was sketchy and left many questions unanswered, especially those concerning the rectilinear propagation of the etheric vibrations. Subsequently the medium hypothesis acquired important support from the eminent mathematician Euler. Leonhard Euler (1707-1783) was known to mathematicians and physicists for his numerous discoveries and to the scientifically interested public for his Lettres Cl une princesse d'Allemagne sur divers sujets de physique & de philosophie. Swiss born, he spent a great deal of his life in St. Petersburg. In 1746 he published his systematic and mathematically founded theory of light and color: Nova theoria lucis et coloris (based on the ideas of his mentor Johann Bernoulli 11). Euler considered ether to be a fluid, extremely fine and elastic substance which conducted vibrations, the frequency of which determined the color. The analogy between light and sound was important to hirn, as to Malebranche. An insuperable objection to the emission hypothesis was, to his mind, the fact that light can pass in all directions through the most compact transparent substances, such as a diamond, just as if they were extremely porous materials. (This was an important argument against the corpuscular hypothesis. The Italian priest Ruggiero Guiseppe Boscovic (1711-1787), a well-known astronomer in his time, produced an interesting counter-argument [7] which sounds quite modem: material is actually empty and consists of punctiform particIes surrounded by a sphere of action). But even Euler could not give a satisfactory explanation of why a vibration in the ether is only propagated as a straight line. Intermezzo: achromatic lenses

Euler initiated an important new development in optics [8]. In 1747 he asked hirnself why the objects we see do not have colored edges, in other words, why the human eye, which is a strongly refractive system, does not exhibit chromatic aberration (at least apparently - Euler's observation was incorrect!). He realized that there were various refractive substances in the eye, and calculated that the color dispersion produced by one substance could be partially neutralized by another substance. This notion of Euler's inspired

93 Dollond to experiments which finally led to the production of achromatic optic systems. John Dollond (1706-1761) was a convinced Newtonian and a capable experimenter in the field of optics. He set out to disprove Euler's suggestion, which implied that Newton had made amistake in the 8th experiment in Book 11 of the Opticks. Newton had described there how a light bundle which passes through two different refractive media does not exhibit color dispersion if it leaves the media without change of direction. That was, according to Newton, the only possibility, because color dispersion was dependent on the refrangibility of the rays and could therefore not occur if the bundle was not refracted. But the emerging bundle did show color dispersion! The dispersion caused by different sorts of glass varied; refraction and color dispersion were therefore not proportional. Around 1750 Dollond successfully produced achromatic lens systems, made of different sorts of glass. Newton's statement that reftecting telescopes were the only way to evade color dispersion, was incorrect. If Dollond had made his discovery shortly after Newton's first publication, it would have been, as Goethe observed, the death-blow to Newton. Newton had stated that prisms did not modify homogeneous white light, but only refracted the rays of heterogeneous white light to different degrees. Now it appeared that the dispersion of colors was dependent on the type of glass of which the prism was made! However, Newton's color theory had been so universally accepted in England, that a veil was drawn over 'Newton's error' and his theory, after redefinition of the term 'dispersion', continued to be generally accepted.

Supporters of the corpuscular hypothesis Jean-Jacques d'Ortous de Mairan (1687-1771), a Parisian scholar, had - after many failed efforts - successfully repeated Newton's eperimentum crucis. Like Newton, he supported the corpuscular hypothesis. In 1717 he published his Dissertation sur la cause de La lumiere des phosphores et des noctiluques. He had studied luminescence and had come to the conclusion that the lightgiving material consisted of the chemical principle sulfur. Luminescent substances, like Bolognese phosphorus and firefties, contain much sulfur. Light corpuscles are ftung out by the turbulence of sulfur. The velocity of the corpuscles determines the color. (We came across this connection between light and sulfur before, by Isaac Vossius, p. 75). Appealing to Pascal's Law, de Mairan rejects Malebranche's vibrations de pression. When sunlight enters a room through a hole in the window shutter the whole room would be uni-

94 formly illuminated; there would be no ditference between day and night were the wave theory correcL In his own words: If the propagation of light takes place through the pressure of light particles on etheric material, then I say that it is impossible that there should ever be night in the universe, or shadows [9]. In Germany the medium hypothesis of Bernoulli and Euler maintained its position for some time, but was exchanged for the corpuscular hypothesis before the end of the century. That was the result of increasing knowledge of photochemical processes, such as the formation of chlorophyll and the blackening of silver salts. lohn Gottfried Voigt writes in 1796 in his article Beobachtungen und Versuche ueber farbiges Licht. Farben und ihre Mischung: The new chemistry demonstrates very clearly that light is something material and that it plays an important role in nature. Thus Newton's system, of which only the mathematical poss1bility was disputed by Euler, 1S completely confirmed by the facts of new chemistry, and it cannot be doubted that it is not mathematics but chemistry that has the right to make decisions about physical reality [10]. He considers that light is a chemical combination of light matter and caloric matter (the substances simples which appear at the top of Lavoisier's table of elements). Light matter itself has no expansive power, caloric matter turns it into a radiant, expansive, (imponderable) liquid. Prisms produce a continuous spectrum: they cause refraction of the light and redistribution of the two elements of which it is composed. The colors are based on different proportions of the two elements. On the grounds of a number of assumptions, Voigt calculates the proportions of light matter and caloric matter in the various colors to three decimal places. Listing these figures serves no purpose, but it is interesting that Voigt in 1796 already had so me idea of the two aspects of light which later - after the discovery of infra-red and ultra-violet and the rise of the energy concept - were able to be defined as 'brightness' and 'radiant energy'.

Conservative Aristotelians The peripatetic tradition, which was represented in the chapter on the Renaissance by Aguilonius and in the chapter on the scientific revolution by Athanasius Kircher and Honoratus Fabri, was not abruptly broken off in the century after Newton. Newton's new color theory could not disconcert a number of conservative lesuits, although it cannot be denied that the scientific content of their publications deteriorated. I restrict myself to three authors, all Frenchmen. In France the lesuits had their own journal: the Memoires

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pour servir l'Histoire des Sciences et des beaux Arts, usually abbreviated to Memoires de Trevoux in reference to the place of publication. It was the ultra-montanistic counterpart to the more progressive Journal des Sr;avans [11] . Lazare Nuguet (died 1752), a Burgundian Jesuit monk, writes an article in the Memoires de Trevoux in 1705, Systeme sur les couleurs, which is worth mentioning. He propounds the old peripatetic thesis that color is produced by the antagonism between light and darkness. With a prism he makes a spectrum, and observes that yellow contains the most light. When he shines artificiallight onto prismatic red he gets a yellowish color. Thus red is nothing other than shadowed yellow! The darkest color in the spectrum is violet: that is c1early blue mixed with some red. Green is a mixture of yellow and blue. Thus only two primary colors remain, yellow and blue, from which all other colors can be composed. Nuguet's theory is a two-color theory, a linear theory in the Aristotelian tradition. In this way Nuguet is aprecursor of Goethe. As his system gives a 'conc1usive explanation' of natural colors, Nuguet does not need to have resource to vibrations in fine material, like Father Malebranche, or rotation of light globules, like Rene Descartes. Nonetheless, one modem element has slipped into Nuguet's system: the role of the retina. As far as sight is concemed the colors are nothing but the disturbance of a greater or smaller number of nerve fibers, which separate from one another in the proportion of the light rays to one another which struck the retina [12]. This statement is not very explicit, but with the assertion that color is 'nothing but' the stimulation of the ends of nerves, even this conservative Aristotelian seems to be infected with the virus of mechanicism and secondary qualities. Louis Bertrand Castel (1688-1757), a Jesuit from Montpellier, owes his farne to an 'ocular harpsichord' which he designed [13]. After being transferred to Paris at the age of 32, he is soon accepted into society and meets celebrities like Rousseau, Montesquieu and Rameau. Although he is interested in the analogy between light and sound, he does not associate hirnself with Newton or Malebranche, but with Athanasius Kircher, his 'first and only mentor', whose explanation of the spectrum he adopts (Fig. 3.6, after Delle Porta). Kircher had said that harmony could exist between colors as well as between sounds, and could have the same effect on the human spirit. Castel challenges Newton's mathematization of nature: Nature shows enough marvels to the eyes of all kinds of people, and there is no need to borrow ambiguous traits from Descartes or Newton to embellish the work of God [14].

In Le vrai systeme de physique generale de M. Isaac Newton (1743) he rejects Newton's cosmology as weIl as his emission hypothesis and his color

96 pcy?4/5.

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Figure 6./. Prismatic refraction according to Newton and 'true' refraction according to Castel (1740). Green (verd) is composed ofyellow Uaune) and blue. At the right the twelve-tone scale of colors.

experiments. The center of the spectrum is white, with red and yelJow on one side and blue and violet on the other, derived from the peripatetic meeting of light and darkness. As green is a mixed color made up of yelJow and blue, and violet is composed of blue and red, there must be three primary colors: red, yellow and blue (Fig. 6.1).

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The great Newton, how could he have been so deeply wrong, and with that apparatus and that fuss that impresses the wise and keeps the universe in admiration and as it were enslaved by its brilliant spectrum of seven colors [15]. Castel introduces his own light theory. White is a single longitudinal vibration; color arises by the addition of a transverse vibration which modifies the white light. The principle of the ocular harpsichord is explained in a letter to Montesquieu of more than 300 pages, published in the Memoires de Trevoux, and in Castel's main work L'optique et les couleurs (1740). Blue is the keynote of the octave, yellow the third and red the fifth. Between each pair of primary colors: blue, yellow and (crimson) red, there are three intermediary colors, so that a 'chromatic' scale of twelve tones is formed: blue, celadon , green, olive-green, yellow, fallow, nacarat, red, crimson, violet, agate, violaceous. There are twelve octaves of increasing degrees of chiaroscuro between black and white. The practical construction of the instrument took years of toil and trouble; it became a complicated instrument with numerous colored glasses illuminated by a hundred candles. The reason that the ocular harpsichord was a failure was not only technical. There is an essential difference between sound and color. The world of colors is sober in comparison with the richness of sound. When an opera fan he ars a recording of a soprano, he recognizes the voice of Maria Callas or Joan Sutherland immediately. Nevertheless, according to the score they are singing the same notes. It is the timbre which is different: the balance of overtones and undertones, the spectrum from which the tone is constituted. Our color vision does not recognize such fine nuances. If two colors lie at the same level in the staff notation of our color vision (Newton's color plane), their spectral compositions may be quite different, but that does not give them a different timbre. Our color vision is limited, even though it includes thousands of colors. Castel was much criticized by Newtonians like De Mairan and Voltaire; the supporters of the wave theory, like Euler, were naturally more favorably disposed. Castel's theory was positively received by the composer Telemann and connoisseurs of art like Diderot. Kant also accepted the possibility of a Farbenkunst, an art of color; it seems as if the master of abstract thought foresaw the abstract art of Kandinsky and Skrjabin, in which color and music are combined! Although I have placed Castel by the anti-Newtonian conservatives, he deserves more than mere anecdotal attention. Not just because he was one of the many color theorists who believed in three primary colors, but rather because he arrived at a SOrt of three-dimensional color scheme, with twelve

98 levels of brightness and at each level a color circle of twelve colors. Even Castel was not immune to the virus of Newton's color system. Jacques Gautier d' Agoty (1716-1785), a worthy engraver in Dijon, was one of Newton's fiercest opponents [16]. He wrote Chroa-genesie ou generation des couleurs, contre le systeme de Newton (Paris 1749). He performed countless experiments to prove that Newton was wrong. Thus he let sunlight fall on a prism after having passed through a narrow copper tube, producing a mixed pattern of diffraction and spectral dispersion. He stated, like Mariotte and Castel, that the spectrum colors arise at the border between light and darkness, and that the principal colors were red, yellow and blue. In France, Gautier's book was refused by the Acad6mie des Sciences. Goethe, Gautier's anti-Newtonian sympathizer, sees hirnself as Gautier's avenger and writes about hirn in his Farbenlehre [17]: In the time that the French Academy repressed this capable man I was a baby of a few months lying in the cradle. Although he had been favored and given a pension by the king, he saw his good name taken from hirn. I am glad to be able to rehabilitate his reputation, to harry his opponents like my own and to fulfill the desire which, because it received no response, he had often expressed: Exoriare aliquis nostris ex ossibus ultor (from our bones an avenger must arise). Gautier's tirade against Newton bore fruit in Italy, where a Professor of Philosophy, Celestin Cominale (1722-1785), wrote a book in two volumes under the title Antinewtonianismus (Napies 1756). In volume I he combated Newton's Opticks and payed much attention to Gautier's experiments. White and black he called primary colors, secondary colors were yellow, blue and red. From a scientific point of view Italy had become a backward country in the eighteenth century; for example, Copemican astronomy had been banned (and remained banned until recently).

Practitioners on the classification of colors Painters, engravers, cartographers and entomologists all feIt the need to cIassify colors. Most of them had become more and more convinced that there were three main colors from which all other colors could be composed, with or without the addition of black and white. One of the first to make a practical scheme of colors and color-naming (without white and black), was Richard Waller (1686), Secretary of the Royal Society. His starting point was the three 'simple colors', red, yellow and blue, from which he produced binary mixtures such as purple and green. In the 18th century the three primary colors belonged to the stock of popular knowledge. Guillaume-Germain Guyot (1724-1800), a scholar and

99 the King's Vicar, wrote in 1769 Nouvelles recreations physiques et mathematiques, a sort of hobbybook for those interested in science. He describes how to build a box with a peep-hole, through which the reader can see that all colors can be put together from yellow, red and blue. The practitioners, unlike Mariotte and Euler, did not usually speculate on the nature of light, the relationship between light and color, or color vision.

Three-color printing Jacob Christoph Le Blon (1667-1741), descendant of a French Huguenot family whieh had fted to Frankfort, is the genius who invented three-color printing [18]. In 1702 he moves to Amsterdam, which is the center of pictorial and graphie art. There he devotes his attention to the mezzotint technique, the technique whieh is best suited to color printing. He uses three copper plates which are successively printed on the same paper after being treated with transparent inks. He follows the idea, which was already generally accepted, of three primary colors: red, yellow and blue, and refers explicitly to Newton's color circle. At the same time he has a sharp eye for the difference between 'material colors' and 'impalpable colors'. In his book Coloritto (1735) he writes: Painting can represent all visible Objects with three Colours, Yellow, Red and Blue; for all other Colours can be compos'd of these three, whieh I call Primitive. And a Mixture of all those Original Colours makes Black, and all Colours whatsoever; as I have demonstrated by my Invention of Printing Pietures and Figures with their natural Colours. I am only speaking of Material Colours, or those used by Painters; for a Mixture of all primitive impalpable Colours, that cannot be feIt, will not produce Black, but the very Contrary, White; as the great Sir Isaac Newton has demonstrated in his Opticks [19]. And thus we may represent a numberless Number of Nudes or Nudities, as different as our heart can des ire, or as our Imagination can fancy or conceive [20] Le Blon is not only a talented artist and a tireless experimenter, but has also an analytical mind. In every color he sees the proportions of the three components and knows how to distribute them over the copper plates. Three color printing, which is now a simple process thanks to photography, was in Le Blon's time extremely expensive and time-consuming. Le Blon makes his first successful color prints in Amsterdam, but moves to London in 1718, where he finds people who are prepared to invest in his business. Le Blon soon realizes the importance of three color printing for medieal illustrations. Le Blon's business is not financially successful and he moves to Paris shortly before his death. There one of his apprentices acquires by intrigue the

100 royal printing rights and continues with his work; it is the aforementioned Jacques Gautier d' Agoty. He had only worked 6 weeks in Le Blon's workshop, but that was apparently long enough for hirn to steal the trade secrets. Gautier presented hirnself as the inventor of color printing; in his Myologie he reproduced beautiful anatomical illustrations. His battle against Le Blon's farne was accompanied by bitter attacks on Le Blon's master, Isaac Newton. The first color triangles

Tobias Mayer (1723-1762) was the first to deve10p a quantitative, metrical color space [21]. He had been a child prodigy who made maps of his hometown as a schoolboy. Later he became an etcher and published his first atlas at the age of twenty-two. Finally he became the director of the astronomical observatory in Göttingen and Professor of Applied Mathematics at the university there. In 1758 he wrote De affinitate colorum, an attempt to give a scientific basis to the coloring of maps. The book was published in 1775, after Mayer's premature death, by the Göttingen physicist and aphorist G.C. Lichtenberg. Mayer constructed a color triangle (Fig. 6.2) with the three main pigments red, yellow and blue, which he mixed together in powder form. Between each of the main colors he placed 11 pigment mixes; inside this framework lay the colors which were formed by mixing all three colors; in the center lay grey (in agreement with Newton, who had also mixed powdered pigments). The tri angle contained a total of 91 colors. By the addition of white and black, each in twelve gradations, Mayer obtained a three-dimensional hexahedron containing a total of 819 colors. The apex of the pyramid above the triangle is pure white, the lower pyramid ends in pure black. There is a striking resemblance between Castel's duodecimal color scheme and Mayer's construction, although Castel's starting point was music and Mayer's was the coloring of maps. The starting-point of both was Newton's color plane; Mayer also took into consideration his rule that a mixed color lay at the center of gravity of a straight line between the two original colors. Both thought that light consisted of three primary colors. Mayer, like Le Elon, realized that there was a difference between mixing pigments and mixing colored lights, without however understanding how great the difference was. Mayer in his color classification underrated the practical problems which presented themselves, due to different physico-chemical properties, spectral reflection, and the transparency of the pigments used. Johann Heinrich Lambert (1728-1777), a famous German mathematician and astronomer and one of the founders of photometry, should be mentioned here although he was essentially a theorist. On the basis of the fact that red, yellow and blue were the artist's main colors, he devised a practical classi-

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Figure 6.2. Tobias Mayer's color tri angle (1758), with mixtures of red (R), yellow (G) and blue (B) in 91 different combinations.

fication and standardization of colors. He also constructed a color pyramid (1772), but in a somewhat different way from his predecessor Tobias Mayer: he did not mix powders alone but used a waxy medium when mixing them. He observed that a mixture of red, yellow and blue in the right proportions produced black. Thus Lambert's color scheme became a single pyramid with black at the center of the base (Plate 2). He did not see how one could expect anything else: yellow was absorbed by red and blue, red by yellow and blue, and blue by yellow and red. That applied to colored glasses too: if one tried to look through three filters, a red, a yellow and a blue one, one saw nothing. With this argument Lambert was actually the discoverer of the principle of subtractive color mixing, a discovery usually attributed to Helmholtz.

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Plale 2. Lambert's color pyramid, with red, blue and as primary calors, and black at the center of the base (1772).

Butterfiies and color-tops

Giovanni Antonio Scopoli (1723-1788) collected butterflies and tried to classify their colors. He deserves special mention, not so much for his results as for the method he used. Scopoli was a prominent North-Italian doctor and naturalist. He was the medical supervisor of the mineworkers in the mercury

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mines in Krain (Slovania) and studied plants and insects there in his free time. His name is perpetuated in the scopolia carniolica, the Krain herb from which the alkaloid scopolamine is won. He described more than thousand insects in his Entomologia carniolica (1763). To standardize the color terms used he constructed a rapidly rotating top on which he placed sectors selected from the colors red, green, blue, white and black. A cinnamon-colored butterfly was given the formula: 4 parts green, 2 red and 2 black. Scopoli had learned the color-top method from Nicolo Poda (born 1723), a Jesuit mineralogist and entomologist from Graz, who had described the Insecta Musei Graecensis in 1761. Poda had perhaps got the idea of a colortop from the work of medieval perspectivists. The Viennese Jesuit 19naz Schiffermüller, who published his Versuch eines Farbensystems in 1772, also intended to dassify his butterflies by color, using Poda's and Scopoli's system, but decided against it: In our study of mixed colors we worked with the better-known method of color mixing used by artists, and expected to confirm our findings (with the color-top). But we found the opposite [22]. Apparently Schiffermüller lacked the scientific instinct to attach special value to an experiment with an unexpected result. His book treats further on 23 different sorts of blue and the pigments with which they can be obtained.

The start of color physiology The mechanistic philosophy of the 17th and 18th centuries postulated that light, which was not actually light, and colors, which were not really colors, stimulated the retina in such a way that light and colors became visible as secondary qualities. Newton had made some vague statements on color physiology, but had not tried to explain how light, which physically speaking showed continuous variation in refrangibility, could give rise to separate, discrete colors. Even in the 18th century, when there was a certain consensus of opinion about the three primary colors, it remained unclear how a conti nuous spectrum of rays, varying in refrangibility in one dimension, could give rise to three primary colors, lying in Newton's two-dimensional color plane. Westfeld and Voigt tried in vain to solve this problem. Christian Friedrich Westfeld (1746-1823), a civil servant from Hanover, was one of the first to ask himself how light affected the retina. In 1767 he wrote Die Erzeugung der Farben, eine Hypothese. His hypothesis was, that the retina was stimulated by a rise in temperature, which caused expansion. Yellow gave rise to powerful expansion, blue to slight expansion. But how was it then possible that a yellow object, seen dose at hand, did not change

104 color when it was further away and could not give as much warmth to the retina? Westfeld complains: I will not explain this hypothesis in more detail, and shall therefore only state what is true, and not what is probable. True is: that the light rays, as simple as they are, must give rise to warmth and expansion in the retina, so that the mind perceives the expansion. But however the colors are to be explained, it will always have to be accepted that whatever produces, for example, the blue color, cannot work with more power than the warmth of a blue light particle [23]. Westfeld was right, but 140 years before the quantum theory he could not prove his point. I.G. Voigt, already mentioned on page 94, believed that the spectrum was continuous and that three basic colors existed. The color sensation, to his mind, was determined by the proportion between the two elements of light (light matter and caloric matter). In 1796 he considered the colors to be secondary qualities: Colours are sensations of the presence of the relative quantities of caloric in light-matter [24]. But how can one, starting from this linear scale, arrive at the subjective color world built up out of three colors? It seemed to be an impossible task. The retina sensitive to three sorts qf light? By circular reasoning (which incidentally implied defection from Newton's theory) it was easier to attain this goal: there are three main colors, so there must be three sorts of light, each of which stimulates certain elements in the retina, thus producing the sensations of the three main colors. This was the argument used by Lomonosov, Palmer, Marat, Wünsch (and, in the 19th century, the English physicist Brewster). Mikhail Vasilevich Lomonosov (1711-1765) was a friend of Euler. They were both members of the Imperial Academy of Science in St. Petersburg. Lomonosow was a colorful figure, both a chemist and a poet. He is regarded as a pioneer of corpuscular physical chemistry. He adopted a mechanistic theory of warmth and even foresaw the concept of absolute zero. His Oration on the Origin of Light. A new Theory of Colour (1756) is in many ways remarkable and interesting. He rejects Gassend's empty space and Newton's emission theory and regards hirnself as an adherent of Descartes' and Huygens' ideas. He assurnes that there are three sorts of ether-particles, large, medium and smalI, with affinity for three chemie al principles: salt, mercury and sulfur respectively. The affinity is mechanical: ether-particles cohere with matter-particles like teeth on a wheel-

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bodies built up by the all-wise Architect and all-powerful Mechanicien [25] The three ether-partic1es, which each follow aseparate path in the retina and the optic nerve, produce in man the sensation of red, yellow or blue. Black objects contain all three chemical principles, white objects only contain inactive, material. A surface which only contains the salt principle absorbs the large ether-partic1es, so that the small and medium-sized particles are reftected and the color green is seen. Lomonosov, who has become very far removed from Newton, refers in his three-color theory to Mariotte. But he also has another source of inspiration: he is the director of a workshop which makes stained glass for windows, and is thus interested in the practice of painting. The painter uses the chief colors and prepares others by mixing; then in nature can there be a greater nu mb er of sorts and ether material for colours than it needs, since nature always follows the simplest and easiest path for its actions [26]? Lomonosov is certainly not the first to think that only three colors exist. But he is the first to produce a physiological hypothesis of color vision on a trichromatic basis. George Palmer (1740-1795) - alias Giros de Chantilly - published his Theory of Light and Colour in 1777 (in English and French) and his Theorie de La Lumiere in 1786. His work has recently been rediscovered and his identity established [27]. He was a glass salesman, specialized in colored glass, with international connections, and also a color theorist. He was not only interested in the theory of color but also in the art of painting and devised a liquid daylight-filter for painters who wanted to paint in artificiallight (1785). Palmer's starting-point was that the spectrum consists of three different rays: red, yellow and blue. When the spectrum is further spread mixed colors also appear. He was more precise than Mariotte in his prism experiments and used a narrower slit, so he saw no white in the spectrum. But his interpretation of the spectrum was the same as Mariotte's and was based on wrong conc1usions drawn from Newton's findings. He also assumed that the surface of the retina is compounded of particles of three different kinds, analoguous to the three different rays of light; and each of these particles is moved by its own ray [28]. It is not known whether Palmer was acquainted with (the Latin version of) the Oration written by the glassmaker and color theorist Lomonosow (although Palmer's text makes it see m probable). In his French artic1e of 1786, Palmer launched an important idea: that colorblindness can arise through loss of color-sensitive elements in the retina. With this idea he anticipates Thomas Young.

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Jean Paul Marat (1744-1793), the French revolutionary and 'friend of the people', was also a doctor with an interest in physics. In 1780 he wrote Decouvertes sur La Lumiere. He placed diffraction at the center of his optics, which he called Pirioptique. Light contains three primary colors, red, yellow and blue. These can be seen (by diffraction) along the edges of a white paper when this is looked at through a prism. Light itself has no color but consists of three sorts of fluid which, via the optic nerve, produce the sensations of red, yellow and blue. With simple circular reasoning and rejection of Newton's work, Marat arrives at a simplistic and poorly-founded physiological threecolor theory. Perhaps his most important cultural contribution remains the fact that he posed for the painter Louis David. Christian Ernst Wünsch (1744-1828, see p. 134) differs from the above authors in that his primary colors are not red, yellow and blue, but red, green and violet. With due respect for Newton he states (1792) that - in spite ofthe gradual differences in refrangibility of the rays which compose white light there are three special sorts of light. I classify the innumerable simple rays from a diffracted light bundle in as many classes as there are simple colors, namely three, and contend that all the rays of the first color produce the sensation of red in healthy eyes, all the rays of the second color the sensation of green and all the rays of the third color the sensation of violet [29]. Wünsch pressed even closer on Young's heels than Palmer. Wünsch also thought that some colorblind peop1e cou1d not see all the primary co10rs; his primary colors, unlike Palmer's, were the same as Young's. But it was Young who took the essential intellectual leap: that it was not light, but the retina, which was trichromatic. Thomas Young Thomas Young (1773-1829) was a comprehensive genius [30]. He could read at the age of two and knew ten languages by the time he was fourteen. He studied medicine, mathematics and physics in London, Edinburgh, Göttingen and Cambridge. At Cambridge he was known as 'the phenomenon Young'. He became a member of the Royal Society as a student, after he had discovered that the lens of the eye was responsible for the focussing of the retinal image of objects seen at close range (accommodation). In a fresh cow's eye he had discovered the activity of the ciliary muscle, and was thus able to make the connection between that muscle and accommodation. In an experiment carried out on his own eye he discovered astigmatism, a refractive error unknown before. These were the most important discoveries made in physiological optics since Kepler.

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Theory of light

Young's greatest achievement was undoubtedly his wave theory of light [31], which he expounded in the Bakerian Lecture at the Royal Academy in 1801. Young was the first Englishman to dare(as delicately as possible) to challenge the posthumous authority of Newton. His attack was directed at the Achilles heel of Newton's light theory: the 'fits of easy transmission and reflection'. During his medical study Young had been interested in sound; he also considered light to be a periodic vibration. The medium was ether, wh ich according to hirn was an indispensable concept in both the light theory and the theory of electricity. He was aware of the phenomenon of sound interference and recognized the same mechanism in Newton's rings. He formulated his interference principle as folIows: The law is, that whenever two portions of the same light arrive at the eye by different routes, the light becomes most intense when the difference of the routes is any multiple of a certain length, and least intense in the intermediate state of the interfering portions; and this is different for light of different colour [32]. On the basis of Newton's precise measurements Young was able to calculate the wavelengths of light of various spectral colors; he found 0.0000266 inch (676 nm) for red and 0.0000167 inch (424 nm) for violet. Young's doubleslit experiment (1807) is his experimentum eruds (Fig. 6.3). When light from a point source falls through two slits a striped pattern of light interference occurs, just like the interference pattern of two waves.

Figure 6.3. Thomas Young's two-slit experiment (1807), demonstrating the phenomenon of interferenee. Two wave erests reinforee eaeh other; a wave erest and a wave trough eaneel eaeh other out. The result is a pattern of lines CDEF, perpendieular to the plane of drawing.

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Young was vigorously attacked by older conservative physicists. The powerful and eccentric Lord Brougham, politician and scholar, wrote in the Edinburgh Review: It is difficult to deal with an author whose mind is filled with a medium of so fickle and vibratory a nature ... this author, in which we have searched without success for some traces of learning, acuteness, and ingenuity, that might compensate his evident deficiency in the powers of solid thinking [33]. Young wrote a reply which no journal would accept for publication. He finally printed a pamphlet himself, but only one copy of it was bought. For years Young's wave theory received little attention, until the French physicist Arago drew a younger colleague's attention to it. This colleague was Fresnel, who rediscovered the interference principle and further elaborated the wave theory of light. Fresnel

Augustin Jean Fresnel (1788-1827), a civil engineer, wrote in his free time Reveries on light. Like Grimaldi he saw light and dark lines round the shadows of objects. He was able to explain the distance between diffraction lines by the interference principle combined with Huygens' principle (Fig. 6.4). He managed to rebut Newton's main argument, that the rectilinear propagation of light was incompatible with a wave theory. Fresnel demonstrated mathematically that the secondary waves, which spread in all directions according to Huygens' principle, cancelled each other out so that only rectilinear propagation and diffraction remained. Fresnel's arguments were so convincing that Newton's corpuscular theory had to suffer defeat. Invisible light

In consequence Voigt's hypothesis, that light consisted of light matter and caloric matter, also had to be abandoned (although it remained popular among chemists, such as the color researcher Wilson, 1855). The concepts of brightness and radiant energy acquired a new significance. '[nvisible light' had already been discovered at the beginning of the century. Friedrich Wilhelm Herschel (1738-1822), who beg an life as a musician but became (as Sir William Hersche!) world famous as the astronomer who discovered the planet Uranus, in 1800 directed light through a prism onto aseries of thermometers and discovered that the mercury went up outside the red end of the spectrum. Johann Wilhelm Ritter (1776-1810), a romantically-inclined German physicist, thought on the grounds of symmetry, that in addition to infrared 'light'

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ultraviolet 'light' must also exist. And in 1801 he was able to demonstrate that silver salts could be decomposed outside the violet end of the spectrum. Light is the source of every power that creates life and activity, a seed that generates all good things that the earth brings forth [34]. For the wave theory this meant that the ether could vibrate over a wider spectrum than that of light alone. The time was not ripe, however, to understand the relation between visible and invisible light (see p. 189).

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Theory of color vision Although Young had been converted to the wave theory of light by the analogy between light and sound, he was weIl aware that there had to be an essential difference between the seeing of colors and the hearing of tones. Young already assumed that tones can be heard because there is aseparate sensory element that resonates with each tone of an accord. Young realized that there was not enough room in the eye for such a system. He wrote in 1802, as a side issue in his treatise on the wave theory of light: Now, as it is almost impossible to conceive each sensitive spot of the retina to contain an infinite number of particles, each capable of vibrating in perfect unison with every possible undulation, it becomes necessary to suppose their number limited, for instance to the three principal colours: red, yellow and blue ... and each sensitive filament of the nerve consists of three portions, one for each principal colour [35]. Young forrnulated here a physiological theory of color vision, based on the current belief in the existence of three main colors. Prom his stay in Göttingen Young was familiar with Tobias Mayer's three-color scheme and Wünsch's book and appears also to have known about Lomonosov's theory. But Young was the first to suggest that the retina split up the continuous spectrum into three parts. That was an extremely revolutionary and daring thought. According to Young there were not three sorts of light, as his predecessors had stated, but three sorts of elementary sensations. Within a year of the Bakerian lecture, Young altered his choice of primary colors. The English physicist William Hyde Wollaston had discovered lines in the solar spectrum (the first 'Praunhofer lines') and thought that they divided the spectrum into four parts: red, yellowish-green, blue and violet. Young for only a short time adopted Wollaston's interpretation of the spectrum as discontinuous, but he maintained his three-color hypothesis and now chose red, green and violet as the primary colors, so that yellow and blue became mixed colors.

In consequence of Dr. Wollaston's correction of the description of the prismatic spectrum it becomes necessary to modify the supposition that I advanced in the last Bakerian Lecture, respecting the fibres of the retina; substituting red, green and violet for red, yellow and blue [36]. It has later become apparent that the choice of three sorts of retinal elements, one sensitive to the middle of the spectrum and the other two to the ends, was a good one, even though it was based on Wollaston's incorrect interpretation of the spectrum.

111 Young's interest in color vision consists mainly of incidental remarks in the sideline of his treatises on the wave theory and light. Young was gifted with genial intuition but did not always follow up his ideas. acute suggestion was always more in the line of my ambition than experimental illustrations [37]. He launched the wave theory of light, but left the further details to Fresnel. He found the key to the hieroglyphics on the Rosetta stone, but the deciphering was done by another Frenchman, Jean-Franc;ois Champollion. Young was not only a lover of science, but also of lively conversation and elegant amusements. Partly because of Fresnel's work, Young's wave theory of light was generally accepted. The idea that light was composed of three separate elements was abandoned. David Brewster, the inventor of the kaleidoscope and an authoritative Scottish opticist, was the last physicist who contested Newton on this point (1822). His theory was disproved by Helmholtz. Young's daring hypothesis of color vision, on the other hand, was not highly rated, partly because of Brougham's and Brewster's opposition. The primary colors red, green and violet were forgotten until Helmholtz, Grassmann and Maxwell gave color science a new impulse in the second half of the century.

VII Classical-romantic color theory in Germany In the preceding pages the 'conservatives' have been repeatedly mentioned, men like Aguilonius, Vossius and Castei, many of whom were lesuits who refused to part with antiquated traditions. The position of the German authors Runge, Goethe and Schopenhauer, who will be considered in this chapter, is very different. They are inspired by ideas which captivated early nineteenth century Germany: classicism and romanticism. They were new ideas, but from the scientific point of view they were reactionary. By referring back to Aristotle's color system, Goethe and Schopenhauer brought color theory to astandstill. Wars and political disintegration in the eighteenth century had caused Germany to fall behind other Western nations. The Napoleonic wars gave the impetus to a serious patriotic consideration of its own national culture. Western rationalism and belief in progress, and the atomistic-mathematical ideas of the scientific revolution, were rejected. In their place came glorification of ancient Greece, which had been rediscovered by Winckelmann, and of Germany's own misty, mythical, medieval past [1]. The object was to find a universal philosophy which, expressed in metaphors and analogies, was directed towards 'the great interdependence of things' [2]. Kanfs doctrine, the culmination of the philosophy of Enlightenment, was changed out of all recognition into the holistic theories of post-Kantian idealism and Schelling's 'philosophy of nature'. A central concept in this natural philosophy was Polarität (polarity), the dualism of body and spirit, of magnetism and electricity, of acids and alkalies, of microcosm and macrocosm. A second basic concept was Steigering (augmentation), the continuous progression of reality to a higher level. Goethe called these two basic concepts 'the two great driving-wheels of nature' and made them the foundations of his color theory. Both Hellenism and romanticism contributed to the proscription of Newton. Newton was the personification of everything that the romanticists and neo- Hellenists disapproved of: mechanicism and atomism, the concept of one uniform law applying equally to the heavenly spheres and the sublunary world. Hegel's re mark that three apples have been fatal in the history of the world is weIl known; the first was picked by Eve, the second was thrown by Paris, and the third fell on Newton's head.

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Runge hirnself is not a color theorist; he is chiefty known for his color classification which does not differ greatly from that of Tobias Mayer (p. 100), although entirely based on the premises of romantic thought. Goethe is the chief figure in this chapter. He constructs a color theory in the neoAristotelian style. Schopenhauer, Goethe's student, formulates a theory which is also Aristotelian and anti-Newtonian, but he does not entirely reject rationalistic ideas and is positively inclined towards Kant's theory of knowledge and the work of the French anatomists and physiologists.

Runge

Philipp Otto Runge (1777-1810), an important romantic painter [3], designed a 'color globe'. His motive for devising a color system is the theory of art, in particular the search for harmony in colors. A key concept in color aesthetics at that time is the 'complementary colors', as they are observed in colored shadows and after-images. Runge, who has no affinity with natural science, derives, as a true romantic, his ideas from religion, mysticism and classical Antiquity. He distinguishes as primary colors blue, red and yellow, and sees an analogy with the Trinity (blue the Father, red the Son and yellow the Holy Ghost), but also with human life: red for love, green for reality, the warm colors yellow and orange for the male passions, the cool colors blue and violet for the female passions. In Runge's color globe (1810, Plate 3) the main colors lie on the equator, equidistant from each other. The secondary mixed colors, violet, green and orange, lie at equal distances between them (Fig. 7.1, in a letter to Goethe, 1806 [4]). The center of the globe is grey, the South Pole is black and the North Pole is white. All colors that lie on the opposite sides of a line through the center are by definition complementary and 'harmonic'; thus orange is by definition the complementary color of blue. Runge who, like Tobias Mayer, placed grey in the center, preferred a color globe to a hexahedron: in a globe all the saturated colors are equally far from the center and white and black have the same rights as the three primary colors. Furthermore, the neo-Hellenistic and romantic Runge considered the globe the ideal figure, as Parmenides and Plato did in old times, and, later, the German mystic Jacob Böhme (1575-1624), who was much admired by Runge. Runge launched his globe at the right moment. He summarized the contemporary knowledge of the multiplicity of colors in an appropriate image, before such a simple system became impossible through the advance of natural science and psychology.

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Piale 3. Runge's color sphere (1810). Left: View of the white pole; right: view of the black pole.

Figure 7.1. Runge's color sehe me (illustration in a letter to Goethe, 1806). The (underlined) primary colors yellow, red and blue are on the circle. The intermediate colors are orange, violet and green.

As is to be expected from a painter, the color globe is a system of surface colors, not of colored lights. Black is the color with least brightness, white (canvas or paper) reftects the maximum amount of light. To modern minds (p. 233) the globe has its shortcomings: the most intense, saturated colors which lie on the equator vary considerably in brightness, blue is dark, yellow is light. Also the same distance on the globe does not necessarily signify the same distance in subjective color differentiation. Runge did not take such factors into consideration, and would have been unable to do so without

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coming into conflict with his own classification principle, which was based on complementary colors and color harmony. Goethe

Johann Wolfgang von Goethe (1749-1832), Germany's greatest poet, published his Farbenlehre in 1810, with its 1400 pages probably the largest book ever to be written about colors [5]. Goethe hirnself considered it his most important work. He dictates to Eckermann, four years before his death: I have no pretentions about anything I have achieved as a poet. Excellent poets have been my contemporaries, excellent poets lived before me and they will also follow after me. But that I am the only person in this century who has the right insight into the difficult science of colors, that is what I am rather proud of, and that is what gives me the feeling that I have outstripped many [6]. There was a good reason for hirn to draw attention to his color theory: in his lifetime he was praised to the skies by the whole of Germany for his poetry, but his Farbenlehre was only coolly received. Goethe's starting-point for his study of colors was pictorial art. He wanted to introduce a system into the use of colors and to gain insight into the laws of color harmony. During his travels in Italy he was struck by the color effects achieved by painters of the Venetian School: blazing, passionate red; cool, tranquil blue. This suggested to hirn a subdivision into warm and cool colors. He was also affected by views of mountains shrouded in a blue veil, and by the red glow of the setting sun. He discovered that there was a system in these two color phenomena and called it the Urphänomen (primordial phenomenon): if sunlight reaches us through a semi-transparent medium, like a thick layer of air when the sun is setting, it becomes yellow, orange or even red. On the other hand, when darkness, like the silhouette of distant mountains, reaches us through a semi-transparent medium it becomes blue or even violet. Opacity - das Trübe - is, as Aristotle had said, an important factor in the production of atmospheric colors. The influence of opacity is particularly clear in nephritic wood. Even in the apparently transparent prism dispersion is caused by opacity (cf. Figure 3.6: Della Porta). Yellow and blue are the basic 'polar' colors produced by interaction between light and darkness. All other colors can be derived from the primordial phenomenon: green arises from a mixture ofblue and yellow; red is augmentation, Steigerung, of yellow; violet is augmentation and red-shift of blue, and purple is a mixture of red and violet. In this way Goethe arrives at a linear color system (Fig. 7.2) in which yellow and blue are the two primary colors. This was contrary to the general opinion of artists, who considered red, yellow

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and blue to be the primary colors. In recognition of the fact that artists, who work with pigments, have a certain right to the three primary colors, Goethe prints at the end ofhis Farbenlehre Runge's lengthy letter (mentioned on page 113) But he states explicitly that: the painter is justified in assuming that there are three primitive colors from which he combines all the others. The natural philosopher, on the other hand, assumes only two elementary colors, from which he, in like manner, develops and combines the rest [7]. Goethe's primordial phenomenon is a mystical concept [8]. It implies a contrast between light and darkness, a contest between two equal powers. It is not man's place to search further than this primeval fact! Eckermann describes a conversation he had with Goethe (1831), one year before his death: then we talked about the great significance of the Primordial Phenomena, immediately behind which one believes that the presence of the Deity can be feIt [9]. Light is to the pantheistic Goethe the revelation of the Divine. Goethe is a light metaphysician in the line of Plotinus, Grosseteste and Ficino. This implies that Light is elevated far above color, which is a sort of pollution of Light by matter and darkness. For Goethe the eye is the organ created by Light in its own image. Look again at the verse taken from Plotinus on page

5!

But the primordial phenomenon is also a scientific challenge. Goethe has read that Newton split white light with prisms. He borrows a prism from a physicist and looks at a white surface with a black border through it. He expects to see spectral colors, but the white remains white! Only the edges of the white surface get a reddish-yellow border on one side and a bluishviolet border on the other. A white area surrounded by a dark border viewed through a prism may thus give rise to the series yellowish-red, yellow, white,

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blue, bluish-violet, but if the white area is reduced, the yellow and blue can be made to overlap and produce green, in the same way that a mixture of blue and yellow paint makes green. Goethe instinctively feels that Newton is wrong: colors do not arise from splitting of light but from interaction of light and darkness. He feels that he has made a great discovery. This problem continues to haunt hirn; even during a battle against French revolutionaries he is more interested in the colors than in the historical event. He reads the treatise on color written by Marat, the revolutionary who was murdered in the bath, who had also studied border colors and had come to almost the same conclusion as Goethe. In his war diary Goethe writes: Light is the most simple, the most undivided and the most homogeneous substance that I know [10]. For decades, until his death, he tries to substantiate this statement. Goethe's Farbenlehre (which had been preceded twenty years earlier by a few preliminary articles on color) is in three parts: didactic, polemica] and historical. The second part is a fierce polemic against Newton, more rhetoric than relevant argumentation. In Goethe's lifetime his publisher tried in vain to scrap the po]emica] part from a new edition of Goethe's work. Goethe represents the great physicist as his personal enemy, the musty man of figures who nailed the living body of the colors to the cross of mechanistic-atomistic thought. The fact is that Goethe was not good at recognizing genius in others: not in Newton, but not in Schubert or Stendhal either. The first chapter of the didactic part, about the 'physiological colors', is the most important theme. Goethe is at his best when describing his own color experiences. Then he is an observant, almost over-sensitive, dilettante with an eye for transient color sensations which others overlook. See, for examp]e, the following passage: I had entered an inn towards evening, and, as a well-favoured girl, with a brilliantly fair complexion, black hair, and a scarlet bodice, came into the room, I looked attentively at her as she stood before me at some distance in half shadow. As she presently afterwards tumed away, I saw on the white wall, which was now before me, a black face surrounded with a bright light, while the dress of the perfectly distinct figure appeared of a beautiful sea-green [11]. Goethe also describes the colored shadows which he saw at sunset on the snow-covered Brocken: In travelling over the Harz in winter, I happened to descend from the Brocken towards evening; the white slopes extending above and be]ow me, the heath, every insulated tree and projecting rock, and all masses of both, were covered with snow or hoar-frost. The sun was sinking towards the Oder ponds. During the day, owing to the yellowish hue of the sun,

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Plate 4. Goethe's color ring (\810). Complementary colors are blue and orange, red and green, yellow and violet.

shadows tending to be violet had already been observable; these might now be pronounced to be decidedly blue, as the illumined parts exhibited a yellow deepening to orange. But as the sun was at last about to set, and its rays, greatly mitigated by the thicker vapours, began to diffuse a most beautiful red color over the whole scene around me, the shadow color changed to a green, in lightness to be compared to see-green, in beauty to the green of emerald [12]. Goethe is here describing subjective colors, complementary after-images, contrasting colors and the like. His precise observations have inspired many later investigators. In the contrast phenomena Goethe saw the etemal formula of life itself. To divide the uni ted, to unite the divided, is the life of nature; this is the etemal systole and diastole, the etemal collapsion and expansion, the inspiration and expiration of the world in which we live and move [13]. Goethe's investigations on the color of after-images resulted in his color circle, which is shown in Plate 4. Green is opposite to red, blue to orange and violet to yellow. It is reminiscent of Runge's color circle with its sixfold symmetry, but Goethe also performed after-image experiments on himself. He worked with pieces of colored paper which are still in existence, but it is

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difficult to say how much they have faded in the intervening years. Runge's scheme may be responsible for the fact that Goethe regarded orange as the complementary color of blue (instead of the more natural yellow), although there are indications that Goethe's blue papers were rather greenish [14]. The phenomenon of colored after-images is now classified under sensory psychology. Goethe speaks of physiological colors and this arouses our curiosity. What sort of physiological explanation would Goethe give for afterimages? Would they arise from exhaustion of the perceptive retinal elements? Such an explanation would never have occurred to Goethe; his explanation is quite different: When the eye sees a color it is immediately excited, and it is its nature, spontaneously and of necessity, at once to produce another, which with the original color comprehends the whole chromatic scale. A single color excites, by a specific sensation, the tendency to universality [15]. It is a remarkable teleological line of reasoning, which one would no longer have expected in the nineteenth century. Goethe's color circle is actually a color ring, a linear succession of colors bent into a circJe. The color plane, one of Newton's great discoveries, was dismissed by Goethe as Quäkelei (nonsense ), and all the colors within his plane were designated 'dirty colors'. Intermezzo: subjective colors before Goethe Subjective effects like after-images and contrast colors had been recognized for a long time before Goethe became interested in them [16]. Aristotle had already described the succession of colors seen if one closes ones eyes after looking directly into the sun. Athanasius Kireher (1646) had compared afterimages with the luminescence of Bologna phosphorus (p. 74). Otto von Guericke (1672), the great mayor of Maagdeburg, described the blue shadows at sunset and saw them as proof of Aristotle's statement that blue arises from a mixture of white and black. George-Louis LecJerc, Count Buffon (1743), the celebrated author of the Histoire naturelle et particu!iere, was interested in couleurs accidentelles like after-images and colored shadows. (Chevreul (1839), page 237, was the first to make a distinction between these two and to speak of successive and simultaneous contrasts.) The Viennese Jesuit priest and physicist Carl Scherffer (1765) wrote an Abhandlung von den zuifälligen Farben and tried to explain after-images on the basis of Newton's theory. When the eye has been looking at the color red for some time it becomes fatigued for that color and thus less sensitive. It therefore sees a mixed color in which red is absent, this is thus the complementary color. An English doctor, Robert Waring Darwin, the father of Charles Darwin, drew exactly the same conclusion in his articJe On the ocular spectra of light and colours

120 (1785). He used the word spectral in the sense of 'ghostlike'; a play upon words which was adopted by Goethe. Darwin made a disc with Newton's colors, which, when rotated rapidly, looked grey. When he removed one of the colored sectors, the color top appeared in the complementary color - the same as the 'reverse spectrum' of an after-image. But were colored shadows really subjective phenomena? Were they not really blue, through illumination by the blue sky? That was what Goethe's somewhat colorless shadow, Johann Peter Eckermann, thought. After long hesitation Eckermann plucked up enough courage to say this to Goethe (February 1829). The great man was furious and accused hirn of heresy; incidentally, Goethe was in the right too. The English physicist Benjamin Thomson, who became lieutenant-general to the Elector of Bavaria and received the title of Count von Rumford, had already demonstrated in 1794 that colored shadows were mainly an optical illusion: when he looked at them through a cylinder they lost their color. Back to Goethe

After his presentation of the subjective color phenomena, Goethe gets into hopeless difficulties in his Farbenlehre when he tries to force the phenomena of prismatic refraction and the colors of real objects onto the Procrustean bed of his Urphänomen, and refuses to admit that border colors can be explained without difficulty by Newton's theory [17]. He is not back in his element until he comes to the description of the 'sensual and moral action of colors': the inftuence of color on moods and feelings, and the effective use of colors in painting. Goethe ends his Farbenlehre with an extensive and interesting history of color theories, in which the emphasis is naturally on pre-Newtonian ideas. He pays great attention to Antiquity and reproduces the complete peripatetic book on colors, credited to Theophrastus, in German translation. He declares that the Greeks knew everything which we recognize as the basis of color theory, and considers it his task to relate how, in the new era, Plato's and Aristotle's ideas received new attention, or altematively came under fire. He writes on Fran~

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specimen the cone population decreases rapidly outside the fovea. Now the cones have been identified as the receptors for clear vision, the following conclusion presents itself: that the cones are also responsible for color vision. It is a fact that colors are only seen in the daytime. That 'day vision' in bright light was later given the name of photopie vision. Then vision at night, scotopic vision, without color or details, remains for the rods. When one observes a weak star in the night sky and directs one's gaze towards it, it sometimes disappears suddenly. Evidently the area adjacent to the central fovea is more sensitive at night than the fovea itself. That is again corroborated by a microscopic specimen: the rod population increases rapidly outside the fovea. A modern illustration by 0sterberg (1935) demonstrates these facts clearly (Fig. 12.9). Day-blindness and night-blindness

The truth of the duplicity theory is confirmed in a surprising manner by pathology: the existence of two hereditary conditions, known respectively as day-blindness and night-blindness. In day blindness or total colorblindness (achromatopsia), a rare hereditary disease, all the symptoms suggest that the cones are not functioning. If one is

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460

500

540

580

620

650

700

740

Wavelength (nm) Figure 13.12. The Land M fundamentals surn up to the luminous efficiency curve (Boynton, 1996).

neural processes. Deuteranopia (internal red-green blindness) and tetartanopia (internal blue-yellow blindness), arise at this level (0). Müller has room in his complicated system for dozens of different color defects and pays much attention to acquired disturbances. About these, much was already known; see, for instance, Köllner's large book (1912). For a long time in the twentieth century they formed a neglected group, but were extensively treated in the book written by Pokorny, Smith, Verriest and Pinckers in 1979. Extremely rare conditions such as tetartanopia (considered to be neural blue-yellow blindness), which, if it exists, is certainly an acquired disorder, deserve much attention from the theorists. Gustav F. Göthlin (1944), physiologist at Uppsala University, published a zone theory in which excitatory and inhibitory processes replace Hering's

207

Figure 13.13. König's fundamenta1s transformed into Herings valence curves. Hell: the photopie 1uminous efficiency curve. Gelb; yellow (Schrödinger, 1925).

assimilation and dissimilation. But in other respects also he had his own ideas about color vision: The authors' view differs with respect of fundamental colours from that of Hering not only by assuming that yellow is the result of a central synthesis of red receptors and receptors from green, but further insofar as it does not assume and cannot assume fundamental red and fundamental green to be complementary colours [11]. Leo M. Hurvich and Dorothea Jameson, two American psychologists, carried out an extensive psychophysical study ofwhite in 1951. They conc1uded that a separate independent white-and-black mechanism had to be postulated, and formulated in the fifties a theory of opponent colors of the Hering type (Fig. 13.15). They performed a quantitative psychophysical investigation (the 'hue-cancellation technique') which resulted in the same sort of 'valency curves' as Hering and Schrödinger had obtained. With their quantitative study they focussed attention in America once more on the theory of opponent colors, especially by psychologists. Their theory is actually a zone theory, but the receptor components here, as by Müller, hardly contribute to the theoretical explanations. At first, Hurvich and Jameson had no objection to Hecht's

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209 closely overlapping components (Fig. 13.19); later they accepted Wright's curves, which were more in agreement with recent physiological findings. Others also succeeded in reproducing Hering's valency curves. Boynton and Gordon (1965) used a simple color-naming method. In the second half of the twentieth century neurophysiology has been an important source of support to the adherents of zone theories. It seems that color vision really has a hierarchical structure, in which signals from the cones are recoded neurally into opponent signals. It is not yet known how this process works, nor to what extent Hering's 'unique' colors playapart in it (see p. 165). Psychophysics is still, more than ever, based on Helmholtz' trichromatic theory, but neurophysiology (which unavoidably lacks the precision of psychophysics) seems to indicate the existence of opponent processes. Electronmicroscopy of the retina

The enormous advances in the microscopic anatomy of the retina are closely associated with the quantum theory. Cajal first raised the microscopical study of the retina to a high standard. His work was elaborated in the first half of the century by Stephen Polyak, in his great books The Retina and The Visual System. But the microscopical work of Cajal and Polyak was limited by the wavelength of light (c. 0.5 thousandth of a millimeter), which does not allow a magnification of more than 3000X. In the twenties Ernst Ruska devised an electronmicroscope in which bundIes of electrons, focussed by ring-magnets, take over the role of light rays. The wave-particle duality of the quantum theory implies that the electrons must have an extremely short wavelength. Because of this, sensation al pictures of the receptors and the neural structure of the retina have been obtained with the electronmicroscope, which easily allows a 100,000-fold magnification. Figure 13.16, from Sjöstrand (1953), shows a greatly enlarged receptor (in this case a rod). The outer segment of the receptor has a number of discs which are the carriers of the visual pigment. Just as impressive is Figure 13.17, taken from Dowling and Boycott (1966), in which the neural connections of the retina are shown. The complexity is enormous. Horizontal cells connect the receptors with each other. The bipolar cells connect with a great number of other cells; they are connected with each other through the amacrine cells. In short, before the signal from the receptor reaches the ganglion cell it must have undergone intensive processing. Study of this figure suggests that a Helmholtz-Hering transformation is quite possible at the retinal level. Much is still unknown of the way in which the

210

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Figure 13.16. Electronmicrograph of three rods. The outer segment (A), connected by a thin cilium B to the cell body, contains a large number of double discs. C: nucleus, D: dendrites of other cells, transmitting the receptor impulse (Sjöstrand, 1953).

211

Figure 13.17. Neural connections in the retina. R: rod; C: cone; MB, FB, RB: bipolar cells; H: horizontal cell; A: amacrine cell; MG, DG: ganglion cells (Dowling and Boycott, 1966).

signals from the optic nerve are further transformed in the visual cortex. The processing there is perhaps even more complicated (see page 243).

212 New facts about color vision defects Heredity

The genetics of color vision defects took definite shape in the first half of the century. The most important recent discovery in the field of heredity is the establishment of the structure of DNA, the carrier of hereditary characteristics, by Watson and Crick in 1953. Subsequently 'molecular genetics' has been developed, the study of how the arrangement of the four nucleotides in the DNA molecule encodes the structure of an organism's proteins. With this technique it was finally possible (p. 220), in 1986, to discover the structure of the visual pigments, and also the genetic defects which prevent the synthesis ofthese pigments or cause them to be made wrong. But we must first consider the basic facts about the heredity of color disorders. In Dalton's and Seebeck's time it was already known that color disturbances are almost exclusively found in males. In 1876 the Swiss ophthalmologist, Johann Fredriech Horner, demonstrated with a large pedigree of deuteranopes, that a deuteranopic man could pass on his condition through his daughter (a 'carrier', who was not colorblind) to a son in the next generation. How that was possible became clear 35 years later when T. H. Morgan discovered sex-linked inheritance in his study of the fruit-fly Drosophila. Perhaps some general information about the various types of heredity may be needed. The hereditary characteristics are to be found in the genes, portions of the DNA which are present in the chromosomes of the cell nucleus. Half of each chromosome derives from the male reproductive cell and the other half from the female cell; they therefore contain two genes for each hereditary feature. If one of the genes is abnormal, a hereditary condition can become manifest if the abnormal gene is dominant to the normal gene. In that case a child can have a manifest abnormality if one of the parents has that abnormality. This sort of heredity occurs in tritanopia. B ut if the normal gene is dominant to the abnormal gene, the child can only get the abnormality if both parents (who are themselves usually normal) have inherited an abnormal gene. This type of heredity, recessive heredity, is seen in total colorblindness. One chromosome, the Y-chromosome, in the male reproductive cell, is smaller than the corresponding X-chromosome in the female cell. This chromosome determines the sex: the woman has a double X-chromosome (XX) and the man has XY. The portion of the X-chromosome that is absent in the Y-chromosome may contain an abnormal gene. If the abnormal gene is recessive, a woman with two X-chromosomes becomes an (almost) normal 'carrier', but a man gets a manifest abnormality. This is the recessive sexlinked heredity of red-green defects. In the case of X-chromosomal recessive conditions men are much more frequently abnormal than women. Where

213

the hereditary abnormalities are localized in the other chromosomes (the socalled 'autosomes'), manifest abnormalities are distributed equally over the sexes. Red-green defects are common. An important study [12] in the 1920s showed that 8% of men have a red-green abnormality, 5% being a deuteranomaly. Only 0.4% of women have a red-green disorder, nearly always a deuteranomaly. During that decade lust and Waaler discovered that protan and deuteran defects were separate conditions (the abnormal genes occupying different positions on the X-chromosome). It therefore follows that the daughter of a protanopic father and a deuteranopic mother has no (noticeable) color disturbance. A gene situated at a certain locus on the chromosome can occur in more than one form. These alternative forms of genes are called 'alleles'. Allelism is an important source of hereditary variation. Various manifestations of individual red-green defects, such as slight protanomaly, severe protanomaly and protanopia are 'alleles', parallel defects, among which the mildest form is dominant to the severest. Thus the daughter of a deuteranopic father and a deuteranomalous mother is a manifest deuteranomal. The various loci of the protan and deuteran defects naturally form a conclusive argument against Hering's theory, which postulated one single form of red-green blindness. Tritanopia BIue or violet blindness was always a problematical element in the trichromatic theory. The cases investigated by König and Dieterici were not homogeneous and the defects were probably for the most part acquired. More information about tritanopia was urgently required. Was it a violet-blindness according to Young's theory or, with a completely different symtomatology, a 'blue-yellow blindness' (a tetartanopia) according to Hering? In 1952 Wright's celebrated article Characteristics ojTritanopia appeared. In a magazine with a wide circulation he had published a colored plate containing a number which was presumably invisible to tritanopes. Everyone who could not read the number was asked to make themselves known. In this way he managed to collect a considerable number of tritanopes. His study put the clinical picture of tritanopia on the map. Tritanopia is very rare; perhaps not more than one case in 50,000. Among other things, tritanopes cannot distinguish blue from green; in the spectrum there are neutral zones in yellow-green and in outermost violet. The convergence point of the isochromes in the color triangle lies close to the violet end of the spectrum, in complete agreement with Young's trichromatic color theory. In 1955 Kalmus proved that the heredity was not X-chromosomal but (irregularly) autosomal dominant.

214 In 1925 Engelking described the c1inical picture of 'tritanomaly'. According to hirn the specific feature of the condition, which appeared to be sex-linked, was that a mixture of spectral green and blue had to be matched in anomalous proportions to produce a certain (desaturated) blue-green. On the grounds of ana10gy with the red-green anomaly, it seemed that tritanomaly should exist, but his cases have subsequently been thought to be incomplete forms of tritanopia [13]. M onochromatism

Typical total colorblindness, in which cone function is completely absent and the visual acuity is therefore very low, is a rare hereditary condition. Even rarer are cases of total colorblindness with normal visual acuity. This condition can have various causes: combinations of tritanopia with protanopia or deuteranopia, post-receptoral or even cerebral disorders. If only the blue cones remain, they are so scarce that the visual acuity is subnormal [14]. As the rods in those exceptional cases can contribute something towards color vision, 'cone-monochromats' can distinguish some color in favorable circumstances.

The visual pigments The rod-pigment

The importance of rhodopsin for scotopic vision was established at the end of the nineteenth century, but nothing was then known about its chemical structure. In 1912 the English biochemist Frederick Gowland Hopkins made a discovery which gave an entirely new direction to medical science. He put rats onto a fat-free diet; they became nightblind and finally died. If a little milk was added to the feed the rats remained healthy. It was an indication of the existence of 'subsidiary foodstuffs', the vitamins. The vitamin which was thought to exist in food which contained fat was given the name 'vitamin A' . Many years later it was found that the nightblind rats had a shortage of rhodopsin in their retinas. Did vitamin A have something to do with rhodopsin? That could not be ascertained until the chemical formula of vitamin A was known; this was discovered in 1931 by the Swiss chemist Paul Karrer. The vitamin was found to be a 'carotenoid', a breakdown product of carotene, the pigment found in carrots. In the first half of this century a 1arge group of research workers concentrated their attention on the rod-pigment [15]. George Wald, an American who

215 had worked for a time with Karrer and later was attached to Harvard University, made the greatest contribution to the analysis of the visual pigments. He discovered in 1934 that a variant of vitamin A formed part of the rhodopsin molecule: rhodopsin was found to be a compound of a medium-sized protein molecule and vitamin A aldehyde (retinal) [16]. The protein was given the name of 'opsin', the retinal group was called the 'chromophore', the colorbearing structure. Later it appeared that the color was largely determined by the structure of the opsin. Wald did not only identify the structure of retinal but also explained (1951) how it was affected by light. In the complicated process of the breakdown (or bleaching) of rhodopsin, the absorption of one photon leads to a change of shape, the photo-isomerization of the retinal molecule [17]. Subsequent chemical changes occur independently of light and lead to the excitation of the receptor and also to the regeneration of the rhodopsin. Within five minutes half of the bleached rhodopsin has been regenerated, within half an hour the process is almost complete. The photo-isomerization of rhodopsin is the start of a whole cascade of chemical events which finally lead to a nerve impulse. Towards the end of the century, DNA-investigation has done much to elucidate the structure of rhodopsin. It consists of seven protein spirals which are incorporated in a fixed position in the double wall of the receptor disc [18]. The molecule protrudes through the disc membrane on both the inner and outer sides (Fig. 13.18). The retinal is situated somewhere in the middle of the rhodopsin molecule. The cone-pigments Although Kühne studied his retinas meticulously he was never able to detect any color at the central fovea. Considering the similarity in structure of the rods and cones, it seemed obvious that the cones must also contain a pigment, and according to the Younge-Maxwell-Helmholtz theory there must even be three sorts of cones, each with a different pigment. But up to the 1960s all cones looked exactly alike, and special histological and physiological techniques failed to reveal any differences. Hecht thought that this was because the differences were minute; Willmer and Segal thought that all cones really were exactly the same. Selig Hecht was professor of biophysics at Columbia University. He did important work in the fields of dark adaptation and the absolute light sensitivity of the eye. In 1930 he proposed a three-component theory of vision, in which the response curves of the three fundamental mechanisms coincided almost completely (Fig. 13.19). He came to this remarkab1e conclusion on the grounds of examination of the visual acuity of normal subjects and dichromats in monochromatic light. The attempt to identify three types of cones

216

Figure 13.18. The structure of rhodopsin. The moleeule has seven spirals located in the disc membrane (indicated by the horizontallines). Each dot represents an amino acid (Hargrave et al., 1983).

with three different visual pigments had been unsuccessful; Hecht thought he could explain why: the pigments were too similar to be differentiated chemically. E. E. N. Willmer, histologist in Cambridge, explained the similarity between the cones in another way. In Retinal Structure and Colour Vision (1946), he dec1ared that the rads, and not the cones (with a yellow pigment that remained after rhodopsin had been completely bleached) functioned as bluereceptor in photopic vision. It was a hypothesis that had also been proposed by König, partially based on the fact that the very center of the fovea (which has no rods) is blue-blind. König's discovery (1897) had been contradicted by other investigators and had been forgotten, until Willmer rediscovered the blue-blindness of the fovea. He thought that the cones were the red-receptors and that green was derived fram a combined cone and rad response. J. Segal, a French physiologist, praposed a still more deviating theory. He stated in his Le mecanisme de la vision des couleurs (1953) that the retina contained more light-sensitive structures than the outer segments of the rads and cones alone. These structures were the pigment epithelium and the synaptic terminals of the cone cells. His theory was dispraved by later discoveries, but the fact that such a completely divergent theory was taken seriously in the scientific world, shows how little was known about the cones

217 1.0

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Wavelength (nm I

Figure 13.19. The three fundamental response mechanisms postulated by Selig Hecht (1930).

and how essential it was to finally demonstrate that there are three different sorts of cones. In the thirties, pigments had already been extracted from the cones of animals with retinas which contained almost exclusively cones. In 1932 the German zoologist G. von Studnitz made an extract of the rod-free retinas of snakes and found a light-sensitive pigment with an absorption curve with three peaks; he thought that his extract contained a mixture of the three hypothetical cone-pigments (Fig. 13.20). In 1937 Wald made an extract of hen's retinas and found a light-sensitive pigment which he called 'iodopsin'. It had an absorption curve with a maximum which roughly corresponded with the photopic sensitivity curve of the human eye. Retinal densitometry Densitometry brought the investigators a step further. In order to measure the concentration of rhodopsin in the living human retina, two English physiologists [19], R.A. Weale and W.A.H. Rushton both constructed around 1950 extremely precise measuring instruments, with which light was projected onto the retina and the retuming light could be analysed by means of an ophthalmoscope and a photo-electric cell. In this way it was possible to determine that light retuming from the fundus contained less purple after bleaching of the rhodopsin present had taken place than before the bleaching, and how great the difference was. The principle is simple, as can be seen in Weale's representation (Fig. 13.21) but the practice is difficult. The same technique was used by Rushton in the 1960s to investigate cone pigment [20]. By dir-

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ecting light onto the central fovea only, he was able to demonstrate that a different pigment was bleached by red light than by blue-green light. In this way he obtained two curves which he also obtained separately, using the same method, in protanopes and deuteranopes. It was a confirmation ofHelmholtz' original theory (Fig. 13.22).

219 COLOR V1510","

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(circles). The curves obtained after a red and after a blue-green bleach coincide, proving that only one pigment is effective in each dichromat (Rushton, 1965).

Cone histochemistry A possible blue-sensitive pigment was not found by densitometry. That was not surprising: the blue-sensitive cones are sparse in the retina. In 1977, when the cones had kept their secret for a century, R. E. Marc and H. G. Sperling succeeded in making the blue-sensitive cones visible under the microscope. They exposed a baboon's eye to blue light and incubated the retina with nitroblue-tetrazolium chloride. Figure 13.23 shows the regular but sparse distribution of the blue-sensitive cones. M icrospectrophotometry In 1964, with great technical ingenuity, spectrophotometric measurements were performed on the outer segments of cones under the microscope. In this way direct evidence was obtained of the existence of three sorts of cones, each with its own pigment. On account of its extreme delicacy, this examination may be regarded as a technical triumph. W.B. Marks, W.H. Dobelle and J.R. MacNichol made their first measurements in goldfish. Figure 13.24

220

Figure 13.23. Selective staining of blue-sensitive cones in baboon retina after exposure to blue light and incubation with nitroblue-tetrazolium chloride (Mare and Sperling, 1977).

shows measurements in human retinas. The data obtained by Paul Brown and George Wald (1964) are very similar. The curves confirm Helmholtz' and König's theory in an attractive and convincing manner. Nevertheless, the structure of the cone pigments was still a closed book.

The structure of the cone pigments The culmination of all this work on cone-pigments and disorders of color vision was recently (1986) achieved by Jeremy Nathans and his colleagues at Stanford University. These molecular geneticists managed to identify and determine the genes which encode the three proteins of the cone-pigments.

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Their starting-point was bovine rhodopsin, the structure of which had been determined by Hargrave et al. in 1983 (Fig. 13.17). They found that the bovine rhodopsin bound strongly with a DNA segment situated on chromosome 3, which was presumably the position of the gene which encoded human rhodopsin. The bovine rhodopsin bound less strongly with three other segments of the DNA; those had to be the genes which encoded the three cone-pigments. Two were found on the X-chromosome, naturally these were the genes for red- and green-sensitive pigment. The third gene was found on the seventh chromosome; considering the autosomal heredity of tritanopia that had to be the gene which encoded the blue-sensitive cone-pigment. At first one gene was found for the red-sensitive pigment; the later discovery of several genes for the L and M cone pigments was unexpected. Comparison of the various genes provided the following information: the human and bovine rhodopsin genes are 90% identical in their DNA sequence; the genes for the red and the green pigment are 43% identical with the gene for the blue pigment, but are 96% identical with each other (Fig. 13.25). Nathans and his co-workers also examined protans and deuterans. Protanopes lack the gene for the L cone-pigment and deuteranopes the genes for the M cone pigment. These results again corroborate Heimholtz' original ideas about red-blindness and green-blindness. Protanomals have no gene for the red-sensitive pigment, but have a hybrid gene for a pigment with sensitivity lying between that of the red- and green-sensitive pigments. Deuteranomals have a similar hybrid gene and sometimes, but not always, a gene for the green-sensitive pigment.

222

BLUE VS RHODOPSIN GREEN VS RHODOPSIN

GREEN VS BLUE

RED VS GREEN

Figure 13.25. Structural comparisons of the four visual pigments. Each black dot represents a difference in amino acids. The red- and green-sensitive pigments are almost identical (Nathans et al., 1986).

In the last years of the twentieth century our knowledge of the visual pigments is racing ahead. Recombinant mammalian cone-pigments can nowadays be produced in vitro in amounts that allow detailed investigation of their mode of action. The evolution of color vision The great similarity between the genes which encode the red and green conepigments suggests that these genes only became differentiated at a late stage of development. It is significant that New World monkeys only have one gene far a long wave absorbing pigment on their X-chromosomes, while Old World

223 monkeys and man have two. It is thought that one gene for a visual pigment split about 500 million years aga into a gene for absorption in the short-wave area and another for absorption in the long-wave area. After the separation of the monkeys into Old World and New World groups, the latter gene will have duplicated itself in the Old World about 40 million years ago into a gene for red-sensitive and a gene for green-sensitive pigment [21]. Thisdevelopment had already been predicted by Christine Ladd-Franklin in the previous century. It is also interesting to reread what Schrödinger wrote on the subject in 1925: When considering the phylogenetic development of a light-perceiving organ, the idea presents itself almost automatically that in the most primitive beginning its function will have been limited to areaction to radiation in a limited range of frequencies. As second stage we may consider that the organ begins to react in a qualitatively different way to two different frequencies within that range. The subjective expression of this is the blue-yellow series with neutral white as transition point. A third, and initially completely analogous, step leads to trichromatism. The split is repeated; yellow is divided into red and green, just as white was earlier divided into blue and yellow. Yellow is for the color-pair red-green what white is for the color-pair blue-yellow: the neutral transition-point. White and yellow are true fundamental colors, but ancestral, not recent. Of the present fundamental sensations, one (blue) dates from the dichromat stage, the other two, red and green, are new acquisitions. That explains why the latter colors are most susceptible to disturbances and 'reversion' and why a disturbance of blue sensation alone, which should be possible according to the three-component theory, as physiological anomaly does not occur [22]. The neurophysiology of the retina

Important contributions to color science in the twentieth century came from neurophysialogy. As same readers may not be familiar with the electrophysiology of the nervous system, a few introductory remarks are given here. It had been known for a long time that life processes were accompanied by electrical reactions [23], even in the eye. Emil Dubois-Reymond, physiologist in Berlin and the actual founder of electrophysiology, discovered in 1849 a weak difference in potential between the front of the eye - the cornea - and the posterior pole. The next step in the electrophysiology of the eye was taken by the Swedish physiologist Fridhiof Holmgren (1865). He discovered that not only this resting potential existed, but also that the galvanometer gave an extra kick if the eye was illuminated. This was the electroretinogram, the

224 electrical response of the eye to light stimulation. Electroretinography has become an important feature of clinical examination, but in the long run it is the differences in potential at the micro-level which have taught us most about the mechanism of vision. Action potentials Electrical examination of the nervous system at the micro-level only became possible after the invention of the thermionic valve, later followed by the transistor, an instrument with which extremely weak electric potentials could be amplified. The fundamental discovery which marked the start of modem neurophysiology was made in 1925 by the physiologist Edgar Douglas Adrian, who was later given the title of Lord Adrian of Cambridge. He was the first to register the electric potentials of individual nerve cells. His discovery was astounding: nerve cells send extremely short electric impulses through their fibers with a frequency of up to 500 per second. In principle, all the impulses are equally strong; the intensity of the nerve signal is determined solely by the frequency and number of the impulses (action potentials). The way in which the action potentials arise was not understood until much later. The nerve cell is a sort of battery, a chemie al source of electricity, in which metallic salts, mainly salts of sodium and potassium, play an important part. The resting cell is in a charged condition. The interior of the cell has a negative potential of about 1/1 0 volt in relation to the extracellular space, from which it is separated by the enveloping and isolating cell membrane. The cell charges itself by means of metabolie activity in its internal organs. Sodium ions are expelled and potassium ions take their place. When a nerve cell receives chemical stimuli from its synapses, the cell membrane becomes locally permeable to sodium ions. 'Depolarization' takes place: the resting potential of the cell is reduced. This alteration is corrected within a thousandth of a second, but is transmitted through the axon as an action potential. The horseshoe crab In Adrian's time, the technical problems involved in registering the action potentials in individual nerve fibers were great. It was found even more difficult to analyse the connection between light stimulation of the eye and action potentials in the visual nerve. In the 1930s an American physiologist, H. K. Hartline, managed to do this (1932). As his experimental animal he chose a primitive crustacean, the horseshoe crab. Like insects, this animal has composite eyes, which are situated in a transparent part of the dorsal shell. The eye can be sawed out, together with a few centimeters of the visual nerve,

225

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intensity of the light stimulus is, respectively, 1000, 100, 10, I unit (Hartline and Graham, 1932).

and examined. The eye is very primitive: each facet receives light from one direction only and can be stimulated individually by a narrow beam of light. Figure 13.26 shows the results of strong and weak illumination of one facet; each facet has a direct connection with the brain in the form of one optic fiber. The visual nerve of the frag

In 1959 Lettvin and his co-workers in America published an article with the arresting title What the frag 's eye teils the frag 's brain. It was a new milestone in the physiology of vision. In the frog various sorts of optic fibers could be distinguished according to their electrical responses. The 'sustained contrast detectors' signalled roughly the positions of patches of light and darkness in the visual field. They might be called safety detectors, because they indicate where a frog can jump to in the event of danger. The response of the 'convexity detectors' was quite different. They were particularly good at signalling small moving objects, thus ideal detectors of flying insects. Finally, 'moving edge detectors' and 'dimming detectors' reported the approach of danger. All

226

1mm

Figure 13.27. The receptive field of a ganglion cel!. At the point of the electrode is a ganglion cello A light stimulus at the center of the field causes the firing rate to increase;