Sir William de Wiveleslie Abney

“ Suppose I, as a normal-eyed person, compare the luminosity of the colour coming through the red slit with that coming through the green slit, and then get the green-blind to do the same, it is evident that any excess in the luminosity as measured by myself over that measured by the green-blind must be due to the green sensation, and we can also see how much red and violet make up his white. ”

“ When we look at small patches of light, we view the colour through the yellow spot in the eye. If, when we have matched the luminosity in the ordinary manner, we turn our eyes some 10° away from the patches, we shall find that except at one place in the green the equality in brightness no longer exists. By a little practice we can make matches of luminosity when the eyes are thus diverted. This will give us a different curve of luminosity, as the yellow spot absorption is absent, and the difference in the heights of the ordinates between the two curves will give us that absorption. ”

“ We can now picture to ourselves that when in the perceiving apparatus in the retina the moving parts—probably molecules or atoms—arrive at a certain amplitude, there is then an impression of light, and that it is quite possible that not only those waves whose motion is exactly of the same period as that of the apparatus will set them in motion, but also those waves which are actually of a very different period. ”

“ If, then, we may presume that they are real colours, the Young theory fails to explain them, and the explanation offered by the Hering theory is much more acceptable, as it propounds the idea that the retina has to be considered as a whole, and that if (say) red light is at work at one part its complementary colour (blue-green) must be felt at another. It would be still more acceptable had it happened that the contrast colours were truly complementary, and if the same action was noticeable when the adjacent part of the retina was not also stimulated. ”

“ All my audience know that when going from bright daylight into a cellar, in which only a glimmer of light is admitted, but little can be seen at first, but that, as the eye “gets accustomed” to the darkness, the surroundings will begin to be seen, and after several minutes what before was blackness comes to be invested with form and detail. ”

“ If we throw the whole spectrum on the screen, and ask a person with normal vision to point out the brightest part, he will indicate the yellow, whilst a red-blind will say the green, and so on. This tells us that the various types of colour blind must see their spectrum colours with luminosity differing from that of the normal eye. The difference can be measured by causing both to express their sense of the brightness of the different parts of the spectrum in terms of white light, or of one another. Brightness and luminosity are here used synonymously. ”

“ The red-blind, be it recollected, sees no red, and if I can place in front of the lens of the lantern some medium which cuts off the red as completely as possible, the audience as well as myself will see the colours approximately as the red-blind would do. ”

“ If he be red-blind he will match in brightness a brilliant red and a feeble white; if he be green-blind he will make a match very similar to normal vision. In the case of the red-blind the slit is then moved into the extreme red, when he will say he sees but one patch of light, whilst the green-blind will see it as a person of normal vision would do. If time permits, the whole luminosity curve may be taken and registered. This is not essential, but interesting for reference. Where complete colour blindness exists, it should be possible to cause him to match a green with a red. ”

“ White again is quite distinct from all the colours in the narrower sense of that word, and black, which we must accept as a sensation, as an affection of consciousness, even if we regard it as the absence of sensation from the field of vision, is again distinct from everything else.53 Hence the sensations caused by different kinds of light or by the absence of light, which thus appear to us quite distinct, and which we may speak of as ‘native’ or ‘fundamental’ sensations, are white, black, red, yellow, green, blue. ”

“ If he be a green-blind person he will tell us the colour is a very pale blue, but when the green slit is covered up and the red uncovered, the match will once more be correct, though the purple, formed by the mixture of red and blue, will appear to him to be a little darker than the white. This is what one would expect, for you must recollect this green in the spectrum he would call white or grey. If then, from what to him is also white, though formed by the rays coming through the three slits, we take away a certain amount of degraded white (green to us) , he must still see white, but darker. ”

“ For my own part, whatever theory of colour sensations may prove to be the right one, I lean strongly to the idea that the cause of vision will be found in chemical199 action, induced by the impact of the different wave-lengths of light falling on sensitive matter. A white substance may absorb all the wave-lengths found in the spectrum, and if it have three sets of molecules, one of which has an atom or atoms vibrating with the same period as the waves of light which show a maximum for one sensation and another for another, and so on, the requirements for the colour sensations are met. ”

“ If the visual sensation they possess be the violet, they will see no light at the extreme red of the spectrum, and very little in the orange. They must match every colour with some shade of grey, for they will only perceive that sensation, in what to ordinary normal eyes is white. We need not detail how those who possess monochromatic vision due to some other sensation would describe the different colours. The diagram will tell us. Suffice it to say, that one colour will only differ from another and from white in brightness. ”

“ There is the fact to remember in making these measures in the extreme red and the extreme violet, that the luminosities of the colours are so small that the illumination of the prism itself, by the white light falling on it, has to be taken into account, since it forms an appreciable portion of the patch of feeble colour. By placing a proper shade of blue or red glass in the front of the collimator slit this white light disappears or becomes negligible, and when the absorption of the coloured glass is known from measurement, we can get a very accurate measure of the extinction of these parts. ”

“ There is one popular objection which is always made against this test, or indeed against any proper test, viz., that the examination is not made under the same conditions which absolutely exist, nor with the very lights which the candidates have to distinguish from one another—that is, the red and green lights. Let me beg of you to remark, that as a mere matter of guessing, the chances are equal that a man would name the light shown correctly. ”

“ With the spectrum test the red will require a greater dilution with white before it will be matched with a green, even if it can be matched at all. Measures of the luminosity at four or five positions in the spectrum, extending from near the extreme red to the blue, will give an unerring criterion of the kind and extent of colour blindness from which they are suffering. The existence of a neutral point in the spectrum is sufficient to indicate that their blindness is of a nature to be dangerous in certain occupations. ”

“ The Young theory tells us that the red or the green sensation suffers fatigue by one or other colour, and that when the eye subsequently rests on a grey surface the other two sensations are chiefly stimulated and cause the complementary colour. It is said that it is easier to produce a negative green image than a negative red image, and the adherents of Hering tell us that this is due to the fact that destructive action is more readily carried out than constructive. ”

“ Surely it is more rational to adopt a theory which does not require such a supposition? If the sensitive matter acted upon by the yellow-blue rays be always of the same chemical composition, the shift cannot occur. It might, perhaps, be allowed that one shift was practicable, but, unfortunately, the shifts must become numerous when the cases of partial colour blindness are to be accounted for, and this would necessitate a constantly varying chemical composition of this matter, and of that acted upon by the red-green rays. ”

“ It has, however, one advantage over most sensations, in that it is a sensation which can be submitted to empyric measurement. The question whether certain phenomena, such as the colours produced by simultaneous contrast, are subjective or real, does not require answering for the purpose that we have in view, but the results recorded may probably help to throw light on it. Colour is an impression caused by the stimulation in the eye16 of some apparatus, that lies near the outer wall of the retina, the effect of the stimulation being conveyed by the optic nerve to the brain. ”

“ 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. ”

“ There must be such a complexity in the sensitive material in the eye, both chemical and physiological, that it may be that the changes induced by light on the sensitive surface of the retina have to be considered from both aspects. The purely chemical change is naturally that to which a physicist is most prone to incline, and his bias must be discounted, as must also that of the physiologist. ”

“ The appearance of the spectrum to the colour blind is as follows:—
From A to E the colour is pure “yellow,” very faint up to D, and reaching a maximum between D and E. From E to one-third beyond F towards G the colour is mixed, varying from “yellow” to “blue,” and becoming neutral or “white” at a point near F. In this part of the spectrum the total intensity, as given by the dotted line, is decidedly less than on either side of it, and near the line F, the retina close to the “yellow spot” is less sensible to light than the parts further from the axis of the eye. ”

“ 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. ”

“ 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. ”

“ The sectors are altered until he says they are both alike, and the observation is satisfactory when he declares the two patches of light are both alike in colour and in darkness. It is curious how misleading the word brightness is to some people who are uneducated. I find it much safer to ask which is the darker colour, rather than which is the brighter. A little patience will always enable you to get a good observation. The place in the spectrum which is the neutral point is now noted. The neutral point is again found, but this time commencing in the blue. ”

“ We are all familiar with the fact that there are three primary colours, and we are naturally led to consider these in the light of the experiments just made. As good a definition as any other of a primary colour is that it is a colour which cannot be formed by the mixture of any two or more25 colours. The original investigators in colour phenomena were the artists, and they found that neither red, nor yellow, nor blue could be formed by any mixture of pigments on their palette, but that all other colours could be made by a mixture of two or more of these three. ”

“ Besides cases of complete blindness due to the absence of one or two sensations on the Young theory, we have other cases, as was said when remarking on the percentage of people who are colour deficient, in which one or even two sensations are only more or less deadened. It has often been said that with the theory provisionally adopted, such cases are difficult to class as red or green deficient. As far as my own observations go, I have never found this difficulty. The luminosity curves of such observers, combined with other indications, give a ready means of classing them. ”