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 1 udes index.

1. Color--Histary. I. Crane, Rabert A. (Rabert Arnold). Licht, kleur, ruimte. Ir. Title. aC494.7.C76 1999 535.609--dc21 98-51580

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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 1 st edition 1999

All Rights Reserved C 1999 Kluwer Academic Publishers Reprinted 2000

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Contents Preface

Color theory in the ancient world 3 Empedocles' four elements and four colors 4 The four-color doctrine 5 Atomism and idealism: Democritus and Plato 6 The empiricism of Aristotle 8 The influence of Plato and Aristotle on science 11 The Hellenistic and Roman era 12 Neoplatonism 16 The end of ancient scholarship 16

" The Middle Ages 17 The early Middle Ages 17 The visual science of the Islamic world 19 The controversy about visual rays 20 Ibn AI-Haytham (Alhazen) 22 Alhazen's theory of vision 22 Colors 25 The refraction of light 26 The science of vision and colors in the prime of the Middle Ages 27 The perspectivists 30

'" The Renaissance 35 Color in the Renaissance 36 Optics in the Renaissance 39 Johannes Kepler 44

IV Light, color and vision during the scientific revolution 50 The scientific revolution 50 Kepler and Galileo 50 Bacon, Gassend and Descartes 51 Descartes and vision 54 New theories of light and color 58 The speed of light 64 The refraction of light 65 The rainbow 68 The chemical colors 71 The color theories of opponents of the corpuscular hypothesis 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

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

VIII Disorders of color vision Dalton Goethe Schopenhauer Seebeck

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

77 78 83 85 86

88 90 91 92 93 94 98 99

100 102 103 104 106 107 108 108 110

112 113 115 119 120 121

126 127 130 131 132

133 133 134 134

Chevreul Voigt, Young and Forbes Helmholtz Mixing spectral colors

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

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

XII Anatomy and physiology of the visual system between

135 135 137 138

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

165 166 170 171

1600 and 1900 175 Anatomy of the retina 175 The neural structure of the retina 178 Anatomy of the visual pathways 179 The duplicity theory 181 Day-blindness and night-blindness 183 Visual pigment 185 The Purkinje shift 186 The photopic luminous efficiency function (V lambda function) 187 Dark adaptation 189

XIII The twentieth century 191 The quantum theory 191 The impact of the quantum theory on the science of vision and

color 193 The physics of color 193 Photochemical processes 194

The quantum theory and the limits of vision 194 The absolute threshold of vision 194 The relative threshold of vision 195 The relative thresholds of color vision 196 Our spectral window and the quantum theory 197 Other important aspects of the twentieth century color theory 198 The further development f color theories 199 The trichromatic theory 200 Luminance and color 202 Zone theories 205 Electronmicroscopy of the retina 209 New facts about color vision defects 212 Heredity 212 Tritanopia 213 Monochromatism 214 The visual pigments 214 The rod-pigment 214 The cone-pigments 215 Retinal densitometry 217 Cone histochemistry 219 Microspectrophotometry 219 The structure of the cone pigments 220 The evolution of color vision 222 The neurophysiology of the retina 223 Action potentials 224 The horseshoe crab 224 The visual nerve of the frog 225 The receptive field 226 Stimulation of the retinal ganglion cells with colored lights 227 Electrophysiology of the cones 228 Opponent processes '229 The advantages of an opponent organization 232 Color and luminance channels from retina to visual cortex 232 Color psychology in the twentieth century 233 The classification of colors 233 The names of colors 234 Contrast 236 The influence of boundaries 237 Color adaptation and color constancy 239 The cortical color mechanism 241 Functional specialization in the areas of the visual cortex 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

Notes

ACknowledgments

References

Index

247 247 248 248 248 249 249 250 251 251

253

261

261

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 (Light-Color-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 addi-tion 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 echan-ization 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 gener-ously 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 immedi-ately 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 at-tempt 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 con-vinced that the eyes shoot fire. The eye can be compared to alantern: the ocu-lar 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 pro-duced 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 vis-ion 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 yel-lowish 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 them-selves. 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 re-turn 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 Parmen-ides 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 poly-gons. 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: Aris-totle 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 real-ity 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 Ar-istotle, 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 qualit-ies. In the case of the eye these are the colors. In order to achieve definitive perception other, less specific, information is often necessary: shape, move-ment, 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 sig-nifies 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 ac-tualization 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 the-ory 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 dark-ness 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 pro-duce grey. The mixing is more like a melting-down process, a chemical reac-tion. 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 diaphan-ous medium is semi-transparent. When we look at light through a semi-transparent 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 op-posite 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 accord-ing 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 con-cords 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 Demo-critus 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 treat-ment of problems. In that quantitative atmosphere physics and cosmology ftourished. For biology and psychology, on the other hand, Aristotelian con-ceptual 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 responsib-ility. In his theories he is closer to Aristotle, and he is a vitalist. He con-siders 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 sci-ence, the Museum, with a large library. Numerous scholars worked in Alex-andria, among whom the geographer Eratosthenes, who made precise meas-urements 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 Hellen-istic period, two scholars should be mentioned who have contributed much to the theory of vision.

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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 contri-butions 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 pre-served. 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 ex-ist 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

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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 vis-ion.) 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 em-peror 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 tempera-ments: 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 phys-iology. 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 he-gemonikon, 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

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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).

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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 late-hellenistic period, inspired by Plato, but also by oriental mysticism. It has something to say about light and color and thus merits attention here. As men-tioned above (p. 8), Plato called light 'The child of Good', but it is Plotinus who really created 'light metaphysics'. According to Plotinus everything ori-ginates 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 fam-ous 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 popular-ized 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 clas-sic 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 con-sidered 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 pros-perous 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 para-doxical. 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.

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lsidore, archbishop of Seville (c. 560-636) is one of those who helped to preserve the ancient scholarship (as handed down by the Roman encycloped-ists 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 pre-cious 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.

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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 inspir-ation, 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 learn-ing. 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 en-couraged scholarship and sent out agents to buy old manuscripts. Syrian, Indian and Greek texts were translated into Arabic. Nestorian-Christian doc-tors 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.

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The controversy about visual rays

AI-Kindi (c. 850) was the first Arab philosopher. At the court of caliph AI-Mamum he was doctor, musician, mathematician and astrologer. He trans-lated 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 ma-terial. 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 be-cause, 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

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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 10-hannitius 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).

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Ibn Al-Haytham (Alhazen)

Ibn al-Haytham (c. 1000), the Arab scholar who made the greatest contribu-tion to visual theory in his time, was a dedicated self-taught man. As was gen-erally the case at that time, the information which he collected was compre-hensive. 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 extramis-sion 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 light-radiating 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

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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!

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The visual information goes straight from the lens to the optic canal. Nat-urally 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).

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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 Al-hazen 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 Aris-totle 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 pre-cedes 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).

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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 compon-ent 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 re-fraction 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 AI-hazen between permanent and apparent colors. The colors of objects, con-sidered 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 New-ton.

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].

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The science of vision and colors in the prime of the Middle Ages

In the eleventh and twelfth centuries there was an important political, so-cial and cultural renaissance in Western Europe. The efficient use of water-power 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. Con-stantinus 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 mon-astery 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 com-plete 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 con-clusion 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), Wil-liam of Conches also considers color to be the specific visual information. For hirn and later authors, visual impressions are assembled from color, the

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'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 Lin-coln. 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'). Gros-seteste believed that Light was the first incorporeal form, that is, the substan-tial 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

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LlBER.X. TRAC.IL OE POTE~TllS

Figure 2.4. The three brain cavities. The anterior cellula phantastica contains the sensus com-munis. 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 wa-ter. He thought that the colors produced were caused by differential refraction and by different degrees of darkness, colors being produced by weakening of

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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 schol-ars. 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 rain-bow 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-

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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 tor-tuously 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 direc-tion. 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:

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VITELLONIS THV-R I N G 0 POL 0 N I 0 P T I.

CAE LIBRI DECEM.

!nLhur,ti,figuris noui. iUuLhati atque autn,infinitis

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Figure 2.7. The formation of the secondary rainbow, from Theodoric of Freibergs De iride (ca. 1300). The refraction of the sun's rays is greatest in the upper raindrop, which appears blue, and least in the lowest raindrop, which appears red (from Crombie, p. 257).

means of concave mirrors on land. Witelo knew that the perfect incendiary mirror was parabolic in shape and went on to describe the actual manufacture of a parabolic mirror out of a concave piece of iron.

Witelo examined the colors which emerge from a prismatic crystal and observed dispersion of rays. He suggested, four centuries before Newton, that colors were produced by differential refraction, but finally retumed to Aristotle's theory that colors are produced by mixing light with darkness, in this case due to opacities in the prism. To produce an artificial rainbow he covered the sides of a hexagonal crystal with opaque wax, leaving a clear side between them, and placed the crystal with its remaining three sides directed towards solar rays admitted through a small hole into a darkened room [17].

Theodoric (Dietrich) of Freiberg (c. 1250-1310) represents another peak in medieval optics [18]. Around 1304 he writes De iride et radialibus impres-sionibus (about the rainbow and colored haloes), in which he describes the passage of rays in the primary and secondary rainbows in the same way as Kamal a1-Din. Theodoric also uses a glass ball as model for a raindrop. It is probable that the two scholars came to the same conclusion at the same time but independently. This is not at all surprising: both had the same teachers: Aristotle and Alhazen. Figure 2.7 shows the passage of rays in the secondary rainbow (against the background of the Aristotelian firmament). Theodoric draws four separate colored raindrops in which the refractive angles are clearly different. To our minds, Newton's theory (the connection between the refran-

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gibility and the color of light rays) is just around the corner. But the idea that light itself consists of colors was impossible to fit into the scholastic line of thought.

Witelo's book Perspectiva was reprinted in 1572, and not for the last time. The book Perspectiva communis, written by another perspectivist, John Pecham (e. 1240-1292) [19], who was also Arehbishop of Canterbury, was even reprinted twiee in the seventeenth eentury. The conc1usion may be drawn that, after the highlight of perspectivism, nothing new was aehieved in op-ties and stagnation oeeurred in the late Middle Ages. An example of this stagnation is the popular book Margarita philosophica, written by Gregorius Reisch, a prior in the Carthusian monastery of Freiburg. 1t is a sort of en-cyc10pedia of all branches of knowledge written in the form of a question and answer dialogue between the discipulus and the magister. The book was reprinted 14 times between 1486 and 1583. The optieal theories of Alhazen, Bacon and Witelo were not mentioned, but Lueretius' simulacra reappeared [20].

111 The Renaissance

The Renaissance, the transitional period between the Middle Ages and the new era, is the period in which natural science (and that is what concems us in this study) begins to loosen its bands with the church and Aristotelian Scholasticism. There are various reasons for this change. In the first place, the rise of a new class of non-ecclesiastical professionals, usually non-academic, full of selfconfidence and unhampered by rigid traditions. They are archi-tects, lens-grinders, artists, cartographers. A representative of this group is the Florentine artist and technician Leonardo da Vinci. A second circumstance which contributes to the secularisation of science is the invention of printing, which made the rapid spread of new ideas possible. In addition, there is the rise of humanism, closely connected with the fall of the East-Roman Empire. Byzantine scholars come to Italy and bring with them the works of ancient au-thors; Plato's work and his high regard for mathematics are found fascinating. The Academy in Florence, a scientific center founded by Cosimo de Medici and rivaling Rome and Paris, becomes the capital of Platonism. Marsilio Ficino (1433-1499) translates Plato and Plotinus and reinstates ancient light-metaphysics. According to hirn, Light is the Image of God, the inspiring, immaterial principle that gives color to the four elements of matter but is itself far superior to color. Pre-Socratic theories also receive new attention in the Renaissance. Pythagoras inspires to an unproductive mysticism of numbers, but also to a numerical, quantitative approach to nature. Bemardino Telesio refers back to Parmenides' ancient philosophy of nature. A1chemy, based on ancient Greek, Egyptian and astrological scholarship, receives new attention in the Renaissance. Paracelsus makes from this hermetic source a new branch of medicine, 'iatrochemistry'.

Although the above developments are detrimental to Scholasticism, Ar-istotelism still holds the universities and ecclesiastical colleges in its grasp. (Anyone who opposed it was threatened with capital punishment in a decree of the Paris parliament in 1624). One conservative scholastic, Franciscus Aguilonius, had much to say about color (1613) and is considered in this chapter.

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Color in the Renaissance

No great changes occur during the Renaissance in the science of color. Even where neo-platonism or another philosophy takes the place of Aristotelian theory, color continues to be determined by the visual objects and any theory which attempts to explain color on the basis of the nature of light is hopeless from the start. I limit myself, therefore, to adescription of the views of the four authors mentioned above.

At the age of eighteen, Leonardo da Vinci (1452-1519) becomes the ap-prentice of the artist Verocchio, from whom he learns painting, sculpture and casting in bronze. Talented and eager to learn as he is, he also studies architecture, mechanics and anatomy. As a painter, he is deeply interested in colors. His aesthetic ideas lie outside the range of this book, but his opinions about the fundamental colors are important. He distinguishes, in addition to white and black, four principal colors: red, green, yellow and blue [1].

The simple colors are six, of which the first is white, although some philosophers do not accept white or black in the number of colors, be-cause one is the origin of all colors and the other is their absence. But, as painters cannot do without them, we include them in the number of the others, and say that in this order white is the first among the simple, and yellow is the second, green is third, blue is fourth, red is fifth, and black is sixth. White we shall name for light without which no color can be seen; yel-low for the earth; green for water; blue for air; red for fire; and black for darkness, which is stronger than the element of fire, for there is no question of density in black through which the rays of the sun can possibly penetrate, and in consequence, illuminate.

This is a classification which is still based on Empedocles' four elements. Nevertheless, he has some doubts:

BIue and green are not in themselves simple colors, because blue is composed of light and darkness, as in the case of the air, that is, it is composed of the most perfect black and purest white. Green is composed of a simple and a compound color, that is, it is composed of yellow and blue.

In Leonardo's time painters more and more practised the technique of mixing pigments (in linseed oil) on the palette. It became common knowledge that yellow and blue produced green. (In the nineteenth century Heimholtz called this type of color mixture subtractive mixture, as opposed to additive mixture, in which the mixing of yellow and blue produce white).

Leonardo will later be praised for his remarkable insight into the spe-cific characters of the four principal colors. This praise is not justified. His predecessor Leon Battista Alberti revived the ancient four-color theory and

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Leonardo only replaced Alberti's ash-like color (cinereum) by yellow [2]. But, at the same time, he succumbed to the temptation of a three-color system.

Bernardino Telesio (1508-1588), a Neapolitan who had great influence in his time, leans on Parmenides and tries to explain