Colour vision

by Sir William de Wiveleslie Abney

CHAPTER X.

Before considering the subject of the extinction of light by other types of colour vision, attention must be called to what has already been brought before you. The various colours of the spectrum have to be reduced to the following amounts before they suffer extinction, the orange light at D being of the value of one candle. (See appendix, [page 217], for complete tables.)

	Reduction in Millionths.	Remarks.
B	10,000 or 1/100	approximately pure red sensation
C	1,100 or 1/909	rather more scarlet
D	50 or 1/20000	orange light
E	6·5 or 1/154000	a green chosen by Maxwell as a standard colour
F	15·0 or 1/67000	beginning of the blue
Blue Lithium	85·0 or 1/11700	a good sample of blue
G	300·0 or 1/3300	approximately pure sensation of violet.

If we make these same colours all of the luminosity of one amyl-acetate lamp (·8 of a candle), we find that the numbers are as follows:—

	Reduction in Millionths.
B	300
C	225
D	48
E	3·3
F	·9
Blue Lithium	1·1
G	1·1

These numbers are remarkable, and we may enforce what they mean in this way. The energy of radiation, and of light also when of ordinary luminosity, varies inversely as the square of the distance from an incandescent body when of small dimensions. But from the above it seems that a white screen receiving the rays from an amyl-acetate lamp in an otherwise perfectly dark place, and having a colour which stimulates the red sensation alone, would be invisible at 58 feet distance, for there would not be enough energy transmitted to stimulate the red perceiving apparatus sufficiently to give the sensation of light. If it were an orange light, such as sodium, of the same luminosity, we should have to move it from the screen 142 feet before the same result was attained. With the green light at E, the distance would be 550 feet, and with the violet the distance would be increased to 1000 feet. The reduction in intensity of white light, which, when of ordinary brightness, is warm, would make it colder, for the red would disappear, and finally the residue of light, just before extinction, would become a cold grey, due to the absence of all colour. The changes in hue that would occur are variable, the variation being due to the loss of colour of the different rays for different amounts of reduction, and then their final extinction. We can place two patches of white light on the screen, and gradually reduce one in intensity, keeping the other of its original value. No one would expect that the two would be dissimilar in hue, as they appear to be when the former is moderately near the extinction value. If we wish to see this perfectly, we should use an extinction box and view it away from the surroundings, which must be more or less slightly illuminated.

It has already been stated that the persistency curve for persons who have normal colour vision is closely the same as that recorded for those who are of the monochromatic type. As this is so, we must expect to find that the persistency curve of these last is the same as their luminosity curve. We put this to the test of experiment and found that our reasoning was correct, for the persistency curve could be almost exactly fitted to it. (See table, pages 217 and 222.) The slight difference between them can be credited to the fact that the whole eye may have been brought into use during the extinction observations, the centre of the eye not being exclusively used. The Figure 31 shows both the extinction and the persistency curves, and also the curve of luminosity for the normal eye.

Fig. 31.

The former were derived from a case P. sent for examination. P. and Q. are brothers, each of whom possesses but one colour sensation, and examination showed that their vision was identical. Mr. Nettleship has kindly given me the following particulars regarding them:—“Their acutes of vision (form vision) in ordinary daylight is only one-tenth of the normal. A younger sister and brother are idiotic and almost totally blind, and in one of these the optic nerves show clear evidence of disease. Hence, the colour blindness of P. and Q. must almost without doubt be considered as the result of disease, perhaps ante-natal, involving some portion of the visual apparatus.” A lack of acuteness of vision would be expected from the small amount of light they perceive compared with normal vision. The fact that two of a family, not twins, possess exactly the same colour sense, and that their extinction curves are entirely different to those suffering from post-natal disease, but similar to those of normal vision, point to their colour blindness as falling in the same general category as that of the congenital type. To this I shall refer again.

Fig. 32.

We may reason still further. With the red- and green-blind the violet sensation is still present, and we may therefore expect that their extinction curves, and consequently their persistency curves, should be alike, and should also agree with that made from your lecturer’s observations. A study of Figures 32 and 33 will tell you that such is practically the case. The former shows the luminosity, the persistency, and the extinction curves of a completely red-blind subject, and the latter the same curves for a green-blind subject (see pages 223 and 224). Both were excellent observers, and their examination was easy, owing to the acquaintance with scientific methods. The accuracy of their results may be taken as unquestionable. Each of them may be taken as a representative of their own particular type of colour blindness. There is an agreement between them at the violet end, but a deviation at the red end of the spectrum. The general form of the curves indicates that the same sensation is extinguished last in all. Now, have we any other criterion to offer? We have. In the first instance, we have the violet-blind person to compare with the others, and also another observer who had monochromatic vision, but whose sensation was different to that of the two monochromatic cases we have so far brought to your notice. We have already stated the peculiarities in colour nomenclature of the violet-blind case. His curve of luminosity for the spectrum was taken ([page 227]), and when compared with the curve of normal luminosity, it became evident that in the red and up to the orange his measures were those which a normal eye would make; but that the luminosity fell off in the green, and finally disappeared to an immeasurable quantity in the violet (see [Fig. 30], curves M and F). If his measures of spectrum luminosity are deducted from those of the normal eye, and the ordinates be increased proportionately to make the maximum difference 100, the figure so produced, when compared with the luminosity curve obtained from the monochromatic observers, was found to be the same, and consequently with the persistency curves above referred to. Endeavours were made to gain a good extinction curve, but the results were not as successful as could be desired; but it was ascertained that, without doubt, his most persistent sensation was not more than 1/180 as lasting as that of the normal eye, or to put it in another way, his green at E was only extinguished when the energy falling on his eye was 180 times greater than that at which it vanished with the normal eye. This plainly teaches us that the missing sensation was that which, when present, is ordinarily the most persistent.

The next is a case of monochromatic vision, which differs from those previously brought before you, and I cannot do better than describe it in the words which General Festing and myself employed in our paper in the “Philosophical Transactions.”

Fig. 33.

The patient (B. C.) had been examined by Mr. Nettleship, who kindly secured his attendance at South Kensington for the purpose of being examined by the spectrum and other tests. [Mr. Nettleship states that this case is without doubt a genuine case of congenital colour blindness, without any trace whatever of disease.] B. C. is a youth of 19, who has served as an apprentice at sea. His form vision is perfect, and he is not night blind. He can see well at all times, though he states that on a cloudy day his vision seemed to be slightly more acute than in sunshine. He was first requested to make matches with the Holmgren wools in the usual manner, with the result that he was found to possess monochromatic vision. He matched reds, greens, blues, dark yellows, browns, greys, and purples together; and it was a matter of chance if he selected any proper match for any of the test colours. Finally, when pressed, he admitted that the whole of the heap of wools were “blue” to him, any one only differing from another in brightness. The brighter colours he called “dirty” or “pale” blue, terms which eventually proved to be synonymous. We then examined him with patches of monochromatic spectrum colours by means of the colour patch apparatus. He designated every colour as “blue,” except a bright yellow, which he called white, but when the luminosity of this colour was reduced he pronounced it a good blue. So with white, as the illumination was decreased, he pronounced it to pass first into dirty blue, and then into a full blue.

Colour discs were then brought into requisition, and it was hard at first to know how to make the necessary alterations, owing to the terms he employed to express the difference which existed between the inner disc and the outer grey ring. By noting that a pale “blue” passed into a pure blue when the amount of white in the outer ring was diminished, and that the inner disc was described as “pale” or “dirty” when the outer ring was described as “a very full blue,” we were enabled to make him match accurately a red, a green, and a blue disc separately with mixtures of black and white.

The following are the equations:—

360 red = 315 black + 45 white.
360 green = 258 black + 102 white.
360 blue = 305 black + 55 white.

With these proportions he emphatically stated that all were good blues, and that the inner disc and outer ring were identical in brightness and in colour.

It may be remarked that this is a case of congenital colour blindness, and that there is reason to believe that some of his ancestors were colour blind.

Before using the discs an attempt was made to ascertain the luminosity of the spectrum as it appeared to him. His readings, however, were so erratic that nothing could be made out from these first observations, except to fix the place of maximum luminosity, the terms “pale” and “dirty” puzzling us as to their real meanings. After the experience with the discs we had a clue as to what he wished to express by pale or dirty blue, which only meant that the colour or white was too bright, and on making a second attempt he matched the luminosities of the two shadows as easily as did P. and Q., the other cases of monochromatic vision. The method adopted was to diminish the white light illuminating one shadow to the point at which he pronounced it a good blue, when a slight alteration in the intensity was always sufficient to secure to his eye equality of luminosity between it and the coloured shadow without his perceiving any alteration in the saturation.

Fig. 34.

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.