Colour Vision

Appendix 201

INDEX 229

PREFACE.

The writer had for some years past, in conjunction with General Festing, and recently as Secretary and Member of the Colour Vision Committee of the Royal Society, carried out a series of investigations on colour vision, and selected that subject when he was invited, in 1894, to deliver the Tyndall Lectures at the Royal Institution.

The brief time allotted for these lectures--an hour on three successive Saturday afternoons--restricted the discussion of some aspects of the question, and confined its treatment in the main to those features most readily explicable by the physicist, and to bringing into notice the latest results which had been obtained from physical experiments. How far the writer has succeeded in the task which he then outlined it is for the reader to determine.

There was no intention in the first instance to publish these lectures. After their delivery, many persons expressed a desire that the information they contained should be rendered accessible to such as were interested in the theory of colour vision, and in deference to that desire the lecture-notes have been re-cast in book form. For the reader’s convenience the matter is now divided into chapters instead of into lectures, and a few additions have been made in the text to explain some of the experimental work to those who have not facilities for its repetition.

The writer has to acknowledge several debts of gratitude. First, to Mr. E. Nettleship, for his kindness in looking over the proofs, and making valuable suggestions whilst the work was passing through the press; and also, as will be seen throughout its pages, for many of the interesting cases of defective colour perception which have been examined by the somewhat novel methods described. Next, the writer’s gratitude is due to Professor M. Foster for the permission he has given to use his admirable description of the Hering theory; and, lastly, to the Royal Society for the permission it accorded to use various diagrams which have served as illustrations to papers which have appeared in its “Philosophical Transactions” and “Proceedings.”

COLOUR VISION.

CHAPTER I.

I must commence this course by saying that I feel the honour that has been done me in asking me to undertake it, connected as it is with the name of Tyndall, whose recent removal from our midst has been deplored by all lovers of science, and by none more than by those who have had the privilege of listening to him at this Institution. It is my duty to speak on some subject of physics, and the subject I have chosen is Colour Vision. I hope it will not be considered inappropriate, since it was Thomas Young, the physicist, whose connection with this Institution is well known, who first propounded a really philosophical theory of the subject. Interesting as it may be to trace how old theories have failed and new ones have started, I feel that for those who, like myself, have but little time at command in which to address you, the historical side of this question must of necessity be treated incompletely.

Colour vision is a subject which enters into the domains both of physics and physiology, and it is thus difficult for any one individual to treat of it exhaustively unless he be a Helmholtz, who was as distinguished in the one branch of science as he was in the other. I am not a physiologist, and at the most, can only pretend to an elementary knowledge of the physiology of the eye, but I trust it is sufficient to prevent myself from falling into any grievous error. I shall try and show you, however, that the subject is capable of being made subordinate to physical methods of examination. I must necessarily commence by a very brief description of those parts of the eye in which it is supposed the seat of vision lies, but in terms which are not too technical. As to the mere optical properties of the eye I shall say but little, for they are not necessary for my purpose, although more particularly adapted to mathematical treatment than the other properties I have to describe.

The eye may be diagrammatically represented as in the figure which is supposed to be a horizontal section of it, the figure being reproduced from Professor Michael Foster’s Physiology.

Scl is the sclerotic coat. Ch the choroid coat, with CP the ciliary process. I is the body of the Iris. R is the retina or inner wall. PE the pigment epithelium or outer wall. L the lens held by the suspensory ligament sp.l. VH is the vitreous humour. ON the optic nerve, ox is the optic axis, in this case made to pass through the fovea centralis, f.c. ]

As far as the perception of colour is concerned, the principal part of the eye which is not distinctly optical--i.e. for the production of images--is the retina, and this it will be seen is in reality an outcrop of the brain, the connection between the two being the optic nerve. Owing to this connection, it is not easy to determine where the seat of colour perception is located; but for the purpose of physical investigation this is not of first-rate importance, nor does it affect the discussion of rival theories except in a minor degree. There are other subsidiary adjuncts in the eye to which, however, I must call attention, as they have a distinct bearing on the apparent intensity of some colours and of the hue that mixtures of others are perceived. The first is what is called the “macula lutea,” or yellow spot, a spot which it may be assumed exists in every eye. It is horizontally oval in form, and is situated in the very centre of the retina, embracing some 6° to 8° in angular measure. It has a brownish or yellowish tint, and the retina at this part is slightly depressed, being bounded by a slightly raised rim. In the centre of this area the retina becomes very thin, having a depression about 1/100 of an inch or ·3 millimetres in diameter, which is named the “fovea centralis,” where it is said that vision is the most acute. This statement can be well credited when we come to consider where the seat of the stimulation of sensation lies. The colour which tints the yellow spot is strongest at the crater-like rim, and fades away centrally and peripherally, and is said to be wholly absent in the fovea centralis.

As the colour of this spot is yellow or brown in the living eye (and that it is probably brown the absorption indicates), it follows that white light passing through it must be deprived of some of its components, though in differing degrees. If the seat of sensation is at the outer layer of the retina, as we shall shortly see must be the case, it will further be seen that when light of any colour which the brown pigment will absorb more or less completely falls on different parts of the oval area, the absorption must vary at each part, and the intensity of the perceived light will be least at the rim and increase centrally and peripherally. As the centre of the yellow spot or fovea is coincident approximately with the point where the axis of the eye cuts the retina, the image of an evenly illuminated object, when looked at directly, must fall on the yellow spot. If, therefore, a patch of such light, the image of which more than covers the spot, be observed, it ought to exhibit a varying brightness of colour corresponding to the strength of the colouring matter which exists at the different parts. This it but rarely does, for habit and constant interpretation of what should be seen prevents the mind from distinguishing these variations; but if the colour brightness, as perceived by the different parts, be submitted to measurement by proper means, the variations in brightness of the image can be readily recognised. A very common method of exhibiting the presence of the pigment is to look at a bright white cloud through a layer of chrome alum. Chrome alum transmits red and blue-green rays. Now as the spectrum-blue rays are those which the pigment will absorb, it follows that the colour of the solution should appear ruddy to the central part of the eye, but on the rest of the retina it should appear of its ordinary purplish colour. At a first glance, and before the eye has become fatigued, this is the case, but the phenomenon soon disappears. Another way of forming an idea as to what the yellow spot absorbs is to throw a feeble spectrum on a white surface and cause the eye to travel along it. If the spectrum be viewed so that it does not occupy more than about 40° of the retina, the movement of the eye will show a dark band travelling along the green, blue, and violet regions as the image of these parts of the spectrum fall on the yellow spot, and their apparent brightness will increase as they fall outside the absorbing area. This proves that an absorption takes place in this area.

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