Colour vision

by Sir William de Wiveleslie Abney

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.

Fig. 1.

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.

Fig. 2.

The retina consists essentially of an inner and outer wall, enclosing matter which is similar to the grey matter of the brain. On the inner wall are the vessels which are connected with the optic nerve. The outer wall is epithelium coloured with a pigment, and it is here that the visual impulses begin, although the rays of light giving rise to them have to pass through the thickness of the retina before so doing. It has already been stated that the light has to pass through the thickness of the yellow spot before a visual sensation is felt in the centre of the field, and the experiments just given offer a fair proof of the truth of the assertion, but there is still another which is perhaps more conclusive. Suppose we have a hollow reflecting ball, as shown in [Fig. 2], and through an orifice A we project a beam of light to B, which meets an obstruction, S, in its path, then A B would be reflected from B to C on a screen C F, and the obstruction S would be marked at C. If another beam from D was directed so as to meet the same obstruction, its presence would be marked at F. Knowing the distance of the centre O of the hollow sphere from F C and its diameter, and measuring the distance between F and C and their respective distances from the axis of the sphere, the distances S B and S E can be calculated. This method is applied in the formation of what are known as Purkinje’s figures. The simplest case is where a beam of light is directed through the sclerotic and transmitted through the lens. Images of the retinal vessels are distinguished as at S, and it is found that they cast shadows, which are seen as dark lines in the glare of the field of vision. The sensation of light must therefore come from behind these vessels, and calculation shows that the seat of the sensation is close to the pigmented inner wall of the retina.

Lying here is a layer of what are known as rods and cones, which have a connection, either actual or functional, with the optic fibres which largely compose the inner wall of the retina, and are connected with the optic nerve. In the yellow spot the cones are much more numerous than the rods, but in the peripheral part the reverse is the case. In the fovea the rods appear to be altogether absent. The total number of cones in the eye has been calculated to be about 3,000,000, of which about 7,000 are in the small fovea. The number of cones will give an idea of their dimensions. This detail has been entered into as it has been supposed that these rods and cones are all-important in translating light-waves into visual impulses. The inner wall of the retina of most human eyes, as has been mentioned, is stained with a black pigment, fuscin, though in albinos it is absent. What its particular use may be is still unknown, for its change by light is so slow that it can scarcely be the cause of vision. In the outer parts of the rods is, however, diffused a substance highly sensitive to light, called the “visual purple,” from its colour, and a theory founded on chemical action, produced by a change in this substance, has been promulgated. Fascinating, however, as such a theory must be, it lacks confirmation. The fact that the cones do not contain it, and that in the fovea are cones alone, renders it difficult to reconcile the theory with the fact that this part of the retina possesses, we are told, the greatest acuteness of sensation as regards light and colour.

The eyes of most vertebrate animals, it may be remarked, have this visual purple, but in those of the bat, owl, hen, and some others the colouring matter seems to be absent. Visual purple is an interesting substance, however, and as it is found in the eye it probably exercises some useful function, though what that function may be is at present unknown. That images of objects can be formed on the retina, owing to the bleaching of this substance, has been proved by experiment. The purple is first changed to a yellow colour, and then passes into white. These “optograms,” as they are called, can be fixed in an excised eye if the retina be detached, and then be treated with a weak solution of alum.

Fig. 3.

Many persons are not aware of the extent of the field of view which the eye embraces. Vertically it takes in about 100°, whilst horizontally it will take in some 145°, more or less. The field is smaller on the nasal than on the temporal side. When both eyes are used, the combined field of view is larger horizontally, being about 180°. The field of view which is common to both eyes is roughly a circle of about 90°. There is, however, a marked difference in the distinctness with which objects are perceived in the different parts of field of view. On the fovea centralis two dots placed so as to subtend an angle of 60″ will be perceived as double. That is to say, if a piece of paper, on which are two dots 1/30 of an inch apart, be placed 10 feet away from the observer, these dots will be seen as separated, whilst dots (in this case they should be black and of good dimensions) placed half-an-inch apart would still appear as one if viewed at the same distance near the periphery of the retina. In the yellow spot the distance apart of the cones is such that they subtend about the same angle as the dots when they are seen separate, viz., about 60″; that is, they are about 16/100000 of an inch apart, and hence may have something to say to the limit of separation. The field for the perception of colour is different to that for light.

The diagrams ([Fig. 3]) will show the fields in a satisfactory manner. The concentric circles are supposed to be circles lying on the retina corresponding to parallels of latitude on a globe, and are not, therefore, equi-distant when seen in projection. To make these circles it must be imagined that we have a bowl, in the middle of which is a thin rod standing upright and passing through the centre, and another rod attached to it at the centre of the sphere of exactly the length of the radius. If this last arm be opened to make an angle of 5° with the fixed rod, and be twisted round like the leg of a compass against the bowl, it will make a circle, the projection of which will give the innermost circle of the diagram; if opened to 10° it will give the next circle, and so on for every subsequent 10°. The lines passing through the centre are 30° from one another, the line stretching from 360° to 180° being a line supposed to be vertical. By means of an instrument called the perimeter, the field of vision for each eye can be measured. With its aid any small object can be made to fall on any part of the retina by directing the axis of the eye to a fixed point and moving the object along one of the diameters. Suppose we wish to ascertain the field for a white object, a small white disc is moved, say, along the horizontal line, and the angles at which the retina just no longer sees it are noted. This gives two points in the field, and they are plotted on the chart—in [Fig. 3] one touches the outside circle, and the other is at an angle of about 65°. The field of vision is next tested along another line, say 300° to 120°, and other points noted and marked on the chart. When the whole circle has been examined, the various points are joined together, and we have the boundary of vision for a white object. The boundaries of the colour perception for (say) small red and green discs are found in the same way. The former is depicted in the left-hand chart and gives the field for the right eye, and the latter with that for white in the right-hand chart for the same eye. It will be noticed that two boundaries are given, one taken at mid-day and the other at 6 p.m. The brighter the colour, the larger is the boundary in both cases, showing that the field of colour vision varies according to the illumination. Now it is difficult from this method of experimenting to determine whether the fields for different colours are the same or differ in extent, as we have no information as to whether the colours themselves which were used were physiologically equal. The only way by which this can be satisfactorily determined is by using spectrum colours each of known brightness and area. (Some preliminary experiments made by myself regarding the colour fields will be found in the appendix, and will be referred to later.) It must not be thought that the various colour boundaries mark the limit at which light is perceived, but only the limit at which colour is seen; outside the boundaries the objects appear of a nondescript colour, to which we shall by-and-by call attention. The yellow spot lies within the circle of 5°, and the blind spot on which no sensation of light is stimulated is shown by the black dot about 15° away from the centre.

I have only attempted to sketch, in unphysiological language, the primary apparatus with which our experiments in colour have perforce to be made.