Colour And Colour Theories

Christine Ladd-Franklin, Ladd-Franklin, Christine

• Publisher: Routledge
• Year: 2013
• Print ISBN:
  9780415209632
• eBook ISBN:
  9781136307171
• Topic: Medicine
• Subtopic: Health Care Delivery

This is Volume VIII of twenty-one in a collection on Cognitive Psychology. Originally published in 1929, the topic of this book, then, is the Ladd-Franklin theory of colour. Dr. Ladd-Franklin has been the first (and is still too nearly the only) physiologist to consider colour always in the light of the development of the colour-sense. This aspect of the subject is frequently reproduced in the present volume.

PART I

I

VISION

VISION ¹ [Lat. videre, to see]: Ger. Gesicht, Sehen; Fr. vision, vue; Ital. visione. The sense whose organ is the eye, whose adequate stimulus is light, and whose nerve is the opticus.

GENERAL

In the ordinary production of visual sensation, several distinct processes in the human organism are involved. In the retina the ether vibrations (which we know to be still ether vibrations when they reach this surface) are transformed into some other form of energy which can be conveyed along the nerves—we know not what form, but at least it must be something very different from light, because vibrations of that degree of rapidity would cause the destruction of delicate nervous tissues. In the occipital lobes of the cortex there takes place, under the influence of this conveyed excitation, some process which is the immediate condition of the visual sensation. Before reaching the cortex, the optic fibres pass through intermediate ganglionic stations (quadrigeminal bodies, optic thalamus), but it is not known that these have any essential part to play in the sensation that enters consciousness—they may have no other function than to effect reflexly the motions of pupil, ciliary muscle (accommodation), convergence, etc., which are essential to effective vision (Fig. 1). When the cortical centres have been destroyed, no visual sensation is possible but the same thing is not true concerning the retina: the basal ganglia and the retina may both be thrown out of action by disease, and sensation may nevertheless persist; as a preceding symptom of migraine, which seems to be due to a spasm in the cerebral, or more rarely the retinal, circulation, and of epilepsy, there are very commonly experienced subjective visual sensations, which are sometimes in the form of rings and balls, like the pressure-phosphenes, or zigzags in incompleted curves (fortification-figures, scintillating scotornata), but which sometimes have the appearance of natural objects or of human figures. These frequently enter the field of vision at one side, and the patient instinctively turns the head and the eyes to follow them: this shows that the cortical process carries with it what is essential to spatial localization without the participation of the retina. But it also shows, as was plainly affirmed by Gowers before the recent work of Flechsig on the subject, that there are secondary cortical centres (association centres, or, as they may perhaps be designated, perception centres) where the immediate data of visual sensation are worked up into complicated forms. This proves that chemical changes in the cortex, although not brought about by excitation coming in from below, suffice to affect consciousness (and with spatial attribute as well as simple sensation quality). On the other hand, there are cases on record of most disturbing visual sensations (rings and balls of colour) due to irritation of the cortex caused by a diseased retina which was entirely blind to light—as was proved by the fact that these disturbances ceased when the eye in question was enucleated.

FIG. 1.—Hypothetical scheme for the optical conducting paths. 00, cortical centre; MM, mid-brain; Ch, chiasm; RR, retinal terminations; ↑, centripetal paths; ↓, centrifugal paths; →, lateral connecting paths.

There are, then, aside from the conducting fibres, four separate stations, in general, in the affection of consciousness by external light—the retina, which is, indeed, not only a neuro-epithelial surface, but also a true nervous centre shifted to the periphery (cephalopods, which have, taken together, all the different nervous layers of the human eye, have some of them in the brain and not in the retina) ¹; the basal ganglia; the primary visual centres in the occipital lobe; and the final association-centres. Each of these may apparently be excited to its characteristic activity by internal sources of activity, in the absence of incoming stimulation from below.

In the lower animals the visual process is certainly of much less complexity than in the human visual organ; all that is essential to such a process is that there should be some form of reaction to the transverse vibrations of the ether [or to the visible electro-magnetic radiations]. Any animal in which a portion of the ectoderm is so differentiated as to be a receptive organ for this form of excitation may be said to possess an eye, whether the reaction to the excitation is conscious or unconscious; in certain of the lower forms of animal life the whole surface of the body is obscurely sensitive to light. Sensations of colour (as well as of form), as they exist in the perfected eye, are modalities of the fundamental luminous sensations, which are without question of rather recent phylogenetic development. Wherever there is an eye with two distinct forms of visual elements, rods and cones, it is probable that there is a sense of colour. Below that, there is no evidence of this aspect of the luminous sensation: many observers have declared that the lower animals have a colour sense, and that they have strong colour-preferences (Graber); but this conclusion is not warranted, for a preference for one region of the spectrum over another may perfectly well be a preference for a particular degree of brightness. Since we have found out that the relative brightness of the different spectral regions is, for ourselves, totally different according as the illumination is faint or bright (the Purkinje phenomenon), there is no reason to infer that animals have any sense for actual colour from the fact that they go from one coloured apartment into another, even though these have been made equally bright for the normal human eye.

The eye is considered to be the most highly developed of the sense organs, not only because of its comparative perfection as an optical apparatus (the lens is a piece of living matter which approaches the regularity of a solid with mechanically perfected curved surfaces), but also because of the number of different forms in which it effects sensible discrimination. The pressure sense, the heat sense, the cold sense, on the other hand, are senses with good local discrimination, but with variation within a single terminal organ for intensity only, without discrimination of quality—we cannot tell whether a given amount of heat comes to us from the infra-red or the red or the yellow rays of the spectrum. In the ear we have discrimination for different objective vibration-rates (the sound-waves) in the form of the different subjective quality attached to notes of different pitch, and to this discrimination is given up the physiological space-discrimination in the auditory organ—namely, the succession of fibres of the basilar membrane; there is left no means of acute local discrimination, and, in fact, in the auditory sense we have no space-discrimination other than by the greater loudness of a sound heard by one ear than by the other.

FIG. 2.—Cones from the different retinal zones (Greef). I, close to the ora serrata; II, 3 mm from the ora serrata; III. half-way between the ora serrata and the papilla IV periphery of the macula lutea; V macula lutea; VI, fovea centralis. V. Graefe u. Saemisch, Hdb. d. Augenheilkunde.

In the eye we have a far more keen and dominating sense for space than in any other sense organ—so much so that the quality which stands pre-eminently in consciousness for space itself is the retinal spatial quality. In this organ, the distribution of rods and cones within the sense organ is the primary physiological intermediary between physical and subjective space; consequently there is left no very exact means for quality-discrimination within the sense, and, in fact, our subjective reactions to differences of vibration-period in light-waves are very inadequate. The whole gamut of light-waves is responded to by us subjectively with only four different sensation qualities—these are, in the order of their development, yellow and blue, red and green. They are the sensations which are produced in their purity by, about, the wavelengths 576 μμ, 505 μμ, 470 μμ and a colour a little less yellow than the red end of the spectrum. For all intermediate wavelengths we have nothing in sensation except combinations of these hues, or colour-blends, as reddish-yellow, blue-green, greenish-yellow, etc., but with this very singular peculiarity, that non-adjacent colour-pairs do not give colour-blends (red and green reproduce yellow, and blue and yellow give white, or grey); were it not for this latter circumstance, the confusion in the response to ether-radiation distinctions would be far greater than it is now. Hence we have no means of determining whether the sensation which we get from wave-length 486 μμ, say, is due to light of that wave-length as an objective cause or to a physical mixture of light of wavelengths 492 μμ and 470 μμ; in other words, our visual organ, as a means of giving us knowledge regarding the radiations reflected from or emitted by objects, is exceedingly inadequate. It follows from this that we can never have, in the play of colours, intricate æsthetic combinations and involutions corresponding to musical compositions in tones. The light-sensation elements are far too simple for that; they are like what we should get from a primitive musical instrument with only four strings.

Provision for vibration-period quality being so inadequate as this, and spatial distribution upon the retina being correlated with the highly developed spatial consciousness of the visual sense, what is the physiological mechanism by which four distinct sources of colour-sense are communicated from retina to brain? Scattered sparsely among the rods (the primitive organ for a non-differentiated luminous sensation) are the cones, which alone, without doubt, provide for the sensation of colour; is a single cone the seat of all four colour-processes, and are all four sets of excitation conveyed from one cone along one optic nerve-fibre to the brain? The physiologists are strongly of the opinion at present that all nerve-fibres convey one and the same sort of excitation, and that conscious distinctions of quality are brought about by different reactions in the cortical cells in which they debouch. But, (1) they have chiefly in mind the simpler structures, where there is nothing against such a supposition (muscle, pressure, auditory sensations)—it is only in the sense for colour that occasion arises for making a different assumption, and hence analogy from other cases is entirely without force; (2) the only anatomical difference that exists between the rods and the cones (for the long, fine, closely pressed together cones of the fovea are not cone-shaped) is exactly this—that communication with the bi-polar cell is on the one hand by a simple knob, on the other by a group of distinct processes (Fig. 3, IV); this suggests that the difference between a black-white series, on the one hand,¹ and sensations in four different tones, on the other hand, has its physiologico-anatomical basis in the provision for disjunct communication between cone and bipolar cell, which is had by means of the several different fibrillæ groups of the cone-base. If it were the case that one fibre could convey one form of excitation only, then the four simple colour-tones would have to be mediated by four different contiguous cones (three, in the original Young-Helmholtz theory, which was based solely upon physical, not at all upon psychological considerations). Helmholtz himself, in the second edition of the Physiologische Optik, has given up this view [for good reason—Isaacson], and regards the several photo-chemical processes which underlie colour as taking place all in a single cone. If one cone mediates one colour only, then a point of purple light, so small that its image falls upon a single cone, should look, as the image passes over the retina, now red and now blue according as it strikes one or another of the visual elements that are fitted to respond to it. This and similar phenomena were announced by Holmgren to occur, and were taken by him as being complete confirmation of the Young-Helmholtz theory; but later experiments from the laboratories of Hering and of König show that such loss of true colour does not take place. Schoute (Zeitsch. f. Psychol., xix, 251) gives good reasons for believing that, in such experiments, the image formed is actually as small as calculation from the constants of the eye would imply. It is true that Graefe (who has examined two perfectly fresh human eyes with the aid of the most modern methods of staining, etc.) finds that the cross-section of a foveal cone is 2 μ; it has usually been given as 4 μ, and hence it is possible that the images formed were not small enough to affect individual cones in the fovea. But, on the other hand, images so small as this cannot fall upon more than one cone at a time in the extra-macular region, and here, too, the phenomenon of incomplete response does not take place. The chief consideration by which the physiologists uphold the similar character of all nerve-fibre transmissions is the fact that only one set of electrical reactions occurs, no matter what nerve is involved; but it would be as safe to infer that any two chemical reactions were the same in kind because they were attended by the same production or abstraction of heat (Hering). The question is of fundamental consequence, and it is hardly worth while to devise colour theories until after it has been settled; nevertheless, the physiologists proper and the physiologists for the eye rest content, seemingly, each in his own belief, with very little counter-discussion. It cannot be too much insisted upon, meantime, that the question as it regards the eye must be settled upon its own merits, and not from analogy with other sorts of nerve-fibre, where the assumption of such complexity is not demanded by the facts. (C.L.F.)

THE EYE [AS. eáge, eye]: Ger. Auge; Fr. œil; Ital. occhio. The end-organ of vision; the mechanism by which vibrations of the luminiferous ether are transformed into the physiological stimulus of visual sensation.

A knowledge of both the anatomy and physiology of the eye may be best attained by studying its development in the animal series and in the vertebrate embryo.

Eye-spots in the Protozoa and lowest Metazoa are collections of pigment granules, commonly brownish or reddish, situated in the ectosarc, or (in the Metazoa) in conjunction usually with the cerebral ganglia, or in groups of epithelial cells on the surface of the body. These cells may occur on a level with the general surface, as in some cœlenterates, be raised into pigmented papillæ, as in the fringes about the mantles and siphons of bivalve molluscs, or drawn below the surface as pigmented sacs or pits, as found in echinoderms, many worms, and in the tips of the tentacles and papillæ of numerous molluscs.

It is often stated to be doubtful whether these simple eye-spots give rise to definite sensations of light; the sensations which accompany their reactions to the rays of the spectrum [if any] may, for aught we know, be sensations of heat, though there are no known instances among the higher animals (where we can more reasonably infer by analogy their sensations from our own) of a special provision for the absorption of heat-producing rays.—It should be borne in mind that dark pigment is rather an accessory than an essential to vision, as is proved by its absence in albinos as well as in many of the lower animals that are still sensitive to light.

The formation of the eye as a pit lined by epithelial cells lends itself to the development of the accessory structures, lens, and other refractive media, iris, and cornea, which render visual images possible. Even a simple pit with pin-hole aperture is capable of forming images on the principle of the simple camera obscura. The secretion of refractive media by the cells forming the pit or sac, and the clearing up of the superficial cells to form a transparent cornea, and their convex thickening to make a corneal lens, are steps which are very easy and which are rapidly taken in the invertebrate series...

Table of contents

• Cover Page
• Half Title page
• Cognitive Psychology
• Title Page
• Copyright Page
• Contents
• Preface
• Introductory (from Professor Woodworth)
• Part I
• Part II Shorter Contributions and Reviews
• Appendix A Eine Neue Theorie Der Lichtempfindungen
• Appendix B
• Appendix C Explanation of Charts
• Appendix D List of Dates and Original Sources
• Glossary
• Index