Colour vision

by Sir William de Wiveleslie Abney

B. C.’s Luminosity and Extinction Curves.

The curve of luminosity, [Fig. 34], is a very remarkable one, being different in character to that of P. and Q., the maximum being well on the D side of E. A great falling off in the luminosity when compared with that measured by the normal eye will be noticed both in the blue and in the red. (For measures see [page 225].) The evidence was therefore presumptive that B. C.’s colour sensation was neither red nor blue, but probably a green.

The next test was made to throw light on this point. He made observations of the extinction of the different parts of the spectrum. His observations were very fair, except on the violet side of F, where they became slightly erratic, but by requesting him to use all parts of his retina to obtain the last glimpse of light, a very concordant curve resulted, as shown in [Fig. 34]. Some of his observations at this part were evidently made with the centre of the retina, for they gave readings which, when the “persistency” curve was calculated, and these observations treated as part of the extinction, agreed with the luminosity curve. We may, therefore, conclude that B. C. has a region in the retina in which there is an absorbing medium corresponding to the yellow spot of the normal eyed. This is diagrammatically shown in [Fig. 34] by the difference in height of ordinates in the persistency (dotted) and the luminosity curves. On the red side of the maximum the two curves are practically identical, except from Scale number 54. At this point it is probable that the white light which illuminated the prism vitiated the readings to some degree. At the violet end something similar, doubtless, occurs, but it is masked by the difference that exists in the extinction by the central part of the retina and that of the whole eye.

It must, however, be remarked that the amount of reduction of the intensity of a ray to produce extinction is very different for B. C. and for the normal eyed, or for the red- and green-blind or for P. and Q. B. C. can bear nearly 200 times less reduction for the rays near E. We have already pointed out that the same is practically the case with M., whom we presume to be violet-blind. We may therefore deduce the fact that the monochromatic vision in this case is of a totally different type to that of P. and Q., and that the last sensation to be lost is the same as that of M. If any violet sensation were present in either, the fact would be made evident by the order of the extinction. The sensation of B. C. is thus apparently the green sensation, though that this particular sensation is exactly the same as that absent in the green-blind is not certain.

The observations made by the different types of the colour blind seem to me to throw great light on the theory of colour vision. They show that when the violet sensation is present, according to the Young theory, the extinction shows its presence; and that where this sensation is absent, the reduction of light necessary to produce extinction is greatly less, and may with great certainty be attributed to a different sensation being the final one to disappear.

CHAPTER XI.

I have so far spoken only of normal, or physiological, colour blindness; a peculiarity, or defect, present at birth, and, as far as is at present known, irremediable, but not associated with any defect of the visual functions, or with any disease or any optical peculiarities. What the nature and seat of this defect may be—whether in the eye or in the sensorium—is at present unknown, although some of the characteristics of the deficiency in colour sensation, I believe, seem to indicate the existence of a special part of the brain endowed with the functions for perceiving colour.

But cases are well known to medical men in which colour vision, normal to start with, fails in greater or less degree in connection with disease. This part of the subject is large and very complex, and requires for its full elucidation an acquaintance with the diseases and disorders of the eye. Many of the phenomena accompanying acquired colour blindness, however, are of great interest to the physicist in his study of colour vision, more particularly in regard to the test of the truth of any particular theory. Through the kindness of several medical men, and Mr. Nettleship in particular, I have had the opportunity of examining by the colour apparatus several types of colour blindness due to disease. One feature, common, I understand, to all, or nearly all, cases, is the presence of some disease of the optic nerve. Defective sight—from loss of transparency of the cornea, the crystalline lens, or other transparent parts of the eye—does not interfere with the perception of colour; nor is true colour blindness, as I am informed, well marked, if present at all, in disease limited to the choroid and retina (see [Fig. 1]). Even in cases of the disease of the optic nerve, medical authorities tell us that great differences exist in the amount of colour defect, and that although the colour defect always goes along with some other serious visual loss, either of form, light, or field, the relation between these several factors of the visual defect is by no means always the same, so far as can be judged by the tests commonly used by ophthalmic surgeons. They tell us that in some cases of disease of the optic nerve, colour vision when tested by the wool test, which will be described shortly, may be almost perfect, whilst the capacity for reading test letters of the alphabet may be extremely bad, and vice versâ. It seems that in some cases these discrepancies cannot be accounted for; but in others the facts can be explained by the limitation of the disease to certain fibres of the optic nerve. Thus, if those fibres which supply the yellow spot region of the retina are alone involved, direct, or central, vision will be much damaged both for form and colour, whilst a little further from the centre of the field, the visual functions in such a case are often quite normal. From what has been said in the opening chapters, this will be understood to be that the colour vision is perfect, but the definition of form more or less imperfect. We are told that cases of this type have long been known and are comparatively common, and often favourable as regards recovery; that the mischief may affect one optic nerve, or both; that when both are diseased the malady is usually due to the action of some toxic substance, and that of all substances known to have this particular effect on the optic nerves tobacco is the most important. I dwell a little on this variety—damage to form and colour sense at the centre of the visual field of each eye from limited, and usually curable, disease of the optic nerve—on account of its interest to myself in the investigations I have made, and also on account of the degree of practical importance which it assumes in connection with the proper reading of signals and coloured lights. These cases of “tobacco amblyopia,” as it is pathologically called, are, of course, always found in men; and it may occasionally happen that such a man, if an engine driver, signalman, or a look-out man on board ship, may still see form sufficiently well to see his signals, but may mistake their true colours. From evidence given before the Committee of the Royal Society on Colour Vision, it appears that the disease causing this type of colour blindness is usually produced by the over-use of tobacco, aided by mental depression and a low state of health. As we have no sumptuary laws, cases of tobacco blindness must frequently occur, and it should be the care of all who have the management of railways or shipping to take measures for preventing persons suffering from this disease from occupying posts which require perfect colour vision in order to prevent the possibility of loss of life.

Congenital colour blindness can at once be discovered, and its possessor be excluded from any post in which normal colour perception is necessary, but with this type a single examination is no safeguard, as it may be developed at any period of a man’s career. The disease is, I believe, a progressive one, and at first is most generally unrecognised, the deficiencies of vision being usually slight at its commencement. It is very often brought to the notice of the sufferer by finding he is unable to read. The words at first seem only slightly indistinct, but later become undecipherable, and as time goes on he is unable to even see the letters. He or his friends then usually think it time to consult the specialist. In tobacco amblyopia the area of insensibility is central, and it may subtend a very small angle or one which covers a considerable portion of the field. I am not aware that it ever extends over it all, but it very generally covers the yellow spot. Now as the eye naturally receives the image on the centre of the retina, it follows that, as the ability to distinguish some colours is absent in that particular region, the patient is practically colour blind, though he can distinguish them on most parts of the retina which are not affected. As regards form vision, it was mentioned in the first chapter that in a healthy eye it is much more acute at the centre than towards the periphery, and instances were given of the angular distances apart that black dots on a white ground were required to be placed to allow their being seen as separate objects when the images were received on the centre of the retina, and at the periphery. Sharp definition may be said to be almost confined to 3° of angular distance at the centre, and most probably this is a happy state of affairs, for if we could see equally distinctly with the whole field of vision, the mind would be distracted from the object which it wished primarily to contemplate.

Bearing in mind the want of definition beyond 3°, and the indistinctness caused by a diseased central area, it will not be surprising to find that form vision in these cases is imperfect throughout, though the colour perception outside such area may be unimpaired. But, practically, men suffering from this disease are colour blind to coloured objects, such as a signal light on a railway or a ship’s light at sea. They may see that there is light at the distant signal or on the bow of a vessel, but will be unable to interpret correctly the colour. The colours which fail to make visual impressions are the reds and greens. Some will distinguish yellow, and very nearly all will distinguish blue with the centre of the eye. If a bright spectrum be thrown on the screen, and a tobacco-blind person be requested to name the colours of the different parts pointed out to him, it is often the case that as his eyes follow the pointer he will tell you that in the extreme red he sees no light, but in the bright red he sees dull white. The bright yellow he will tell you is a pale yellow or white, according as his case is a moderate or bad one; the green he will call white, and the blue and violet he will designate correctly. At the same time that his eye is turned away to another colour, he will see the true colour of the part of the spectrum which he has just incorrectly named, but it will disappear again as he turns his eyes back again. This tells us that his sense of colour is apparently unaffected outside the diseased area.