CHAPTER V.
The readiest means of investigating the stimulation of the different sensations necessary to produce colour is evidently by eyes in which one or two sensations are absent, and this applies not only to the Young theory, but also to that of Hering.
In former days, not much more than a century ago, the existence of colour blindness, as it is now named, was a matter of great curiosity, and in the Philosophical Transactions of the Royal Society of 1777, the case of a shoemaker named Harris is described by a Mr. Huddart, who travelled all the way from London to the Midlands on purpose to see if all the alleged facts regarding the patient were true. Harris mistook orange for green, brown he called black, and he was unable to distinguish between red fruits and the surrounding green leaves. At first, colour blindness was called Daltonism, from the fact that the great chemist Dalton suffered from it, and investigated the variation which he found existed in his vision from that of the majority of his fellow-creatures. It was in 1794 that Dalton described his own case of colour blindness. He was quite unaware of his defect till 1792, when he was convinced of its existence from his observations of a pink geranium by candle-light. “The flower,” he says, “was pink; but it appeared to me almost an exact sky-blue by day. In candle-light, however, it was astonishingly changed, not having any blue in it; but being what I call a red colour which forms a striking contrast to blue.” He goes on to remark that all his friends except his brother (mark this relationship), said: there was not any striking difference in the two colours by the two lights. He then investigated his case by the solar spectrum, and became convinced that instead of having the normal sensations, he only had two or at most three. These were yellow, blue, and perhaps purple. In yellow, he included the red, orange, yellow, and green of others, and his blue and purple coincided with theirs. He says, that “part of the image which others call red, appears to me little more than a shade or defect of light; after that, the orange, yellow and green seem one colour, which descends pretty uniformly from an intense and a rare yellow, making what I should call different shades of yellow. The difference between the green part and the blue part is very striking to my eye, they seem to be strongly contrasted. That between the blue and purple much less so. The purple appears to be blue much darkened and condensed.”
Dalton said a florid complexion looked blackish-blue on a white ground. Blood looked like bottle green, grass appeared very little different from red. A laurel leaf was a good match to a stick of sealing-wax. Colours appeared to him much the same by moonlight as they did by candle-light. By the electric light and lightning, they appeared as in day light. Some browns he called red, and others black.
Mr. Babbage, in Scientific London (1874), gives an account of Dalton’s presentation at Court.
Firstly, he was a Quaker, and would not wear a sword, which is an indispensable appendage to ordinary Court-dress. Secondly, the robe of a Doctor of Civil Laws was known to be objectionable on account of its colour—scarlet, being one forbidden by the Quakers. Luckily, it was recollected that Dalton was affected with that peculiar colour blindness which bore his name, and that as cherries and the leaves of a cherry-tree were to him of the same colour, the scarlet gown would present no extraordinary appearance. So perfect evidence was the colour blindness, that the most modest and simple of men, after having received the Doctor’s gown at Oxford, actually wore it for several days in happy unconsciousness of the effect he produced in the street. The rest of the description we need not reproduce. Both the above cases we shall see shortly come under the category of red-blindness in the Young theory. Recent investigations tell us that such colour blindness is by no means rare, nor can it have been then. Statistics, derived from carefully carried out examinations made in various parts of the world by an approved method of testing, show that about four out of every hundred males suffer from some deficiency in colour perception, but that so far as the more limited statistics regarding them are to be depended upon, only about four out of every 1000 women suffer in the same manner.
Colour blindness in a healthy subject is usually hereditary, and is always congenital. It is curious to trace back in some instances the colour blindness, where it is to be found, in a family. It often happens that colour blindness—as the gout is said to do—skips a generation. This is usually traced to the fact that the generation skipped is through the mother. Thus, the maternal grandfather may be colour blind, as may be the grandsons, but the mother will very frequently have perfectly normal vision for colour. On the other hand, the paternal grandfather may have defective colour perception, and this may be inherited both by the grandsons and the father. The remark made by Dalton regarding his brother’s eyesight points to the fact that his own colour blindness was probably hereditary. Deaf mutes, Jews and Quakers, seem to be more liable to colour blindness than other people, statistics giving them 13·7, 4·9, and 5·9 as the percentages. It may be well to point out that the deficiency in colour perception to which we are alluding is totally distinct from that which may arise from disease. This last form has such marked characteristics of its own that it can at once be distinguished from the congenital form.
Of the four per cent. of males who suffer from congenital colour deficiency of vision, a large number are not totally lacking in any one or more colour sensations. Those in which one sensation, on the Young theory, is entirely missing are called “completely red-, green-, or violet-blind,” whilst those in which the sensation is but partially deadened are called “partially red-, green-, or violet-blind.” When two sensations are entirely absent, and such cases are very rare indeed, they are generally said to have monochromatic vision; that is, every colour to them is the same, as is also white, the only distinction between any of them being the superior brightness of some over others. The best illustration of this form of colour vision is perhaps to say that the retinæ of such people have the same characteristics in regard to sensitiveness as has a photographic plate, the resulting prints in black and white representing what it sees in nature. When we have to adopt the terms used by the followers of Hering’s theory—the theory which obtains most followers amongst the physiologists, since it endeavours to explain colour vision in a physiological way, though it fails to meet all the requirements of the physicist—we should restrict our terms to red-green and yellow-blue blindness, still perhaps retaining the term monochromatic vision for the rare cases specified above. As we must employ some terms to express our meaning, we shall in these lectures adopt those of the Young theory.
Now taking a red-blind person and examining him with the spectrum, we find that he sees no light at all at the extreme limit of our red, and only when he comes to the part where the red lithium line marks a certain red does a glimmer commence; he then sees what he may call dark-green, or he may call dark-yellow. When questioned about what to us are greens he also calls them green or yellow, some being bright, others saturated hues, and others again paler. When he gets to the bluish-green he calls it grey, and will say it is indistinguishable from, and in fact will match with, a white degraded in tone. From this point he will say he sees blue, near F pale-blue, and in the violet dark-blue. Too much importance must not be attached to the nomenclature adopted by the colour blind. They have to take the names of the colours from the normal eyed. Yellow objects are generally brighter than red, and having annexed the idea that what to them is bright red is called yellow, they give it that distinguishing name. His limit of vision at the violet end will be the same as the majority of mankind, but it will be considerably shortened at the red end. The point in the spectrum which he calls grey is an important point, and corresponds to the place where the violet and green curves cut in [Fig. 16]. This point can be very accurately determined by placing alongside the colour patch A ([Fig. 6]) the white patch, which is reduced in brightness as required by rotating sectors. As the slit is moved along the spectrum it will eventually reach a point where he will say both patches of light are exactly similar in hue. To the normal eye one will be white and the other the kind of green indicated above (see frontispiece).
If a similar examination be made of the green-blind, the red end of the spectrum will be called red or yellow, but the spectrum itself will be visible between the same limits as it is to the person who has the normal sense of vision. A grey stripe will be seen in the spectrum, but in this case it will be a trifle nearer the red end of the spectrum than the point which the red-blind calls grey; from this point to the extreme violet, the green-blind will name the spectrum colours similarly to the red-blind. The part of the spectrum where grey exists to the green-blind is even more important than that part at which it exists to red-blind, for it marks the place where the red and violet curves cut each other in [Fig. 16], and is in the majority of cases the place in the spectrum where to the normal eye the green sensation is unmixed with any sensation except that of white, as quite recently explained. This green evidently is the colour which is most usefully employed in making colour mixtures in order to obtain the three sensation curves of the Young theory, since white can be added to the colour matched. To avoid verbiage, we shall call the points where the red- or green-blind see a grey in the spectrum their neutral points, and the grey they see at those points their neutral colours. The three curves we shall call the red, green, or violet curves, and the slits, when placed in the red, green, or violet of the spectrum, as the red, green, and violet slits.
We have already mentioned the case of those who possess monochromatic vision, and shown in what respect they will differ in their description of the spectrum from those more common cases of defective vision. 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.
It is a very remarkable fact how many people who are defective in colour vision pass through a good part of their lives without being definitely aware of it. It is very doubtful whether, in the majority of cases, they themselves discover it. They may quite possibly attribute the descriptions of colour which they hear, and which appear to them absolutely false or meaningless, as due to mental or moral defects in their friends. I have had two cases of this recently. One was a gentleman of seventy-four, who had no conception that he had anything but normal colour vision; his daughters, however, had a suspicion that something was not quite right in it, and after a good deal of persuasion brought him to me to examine. The first mistake that he made was to state that he was sitting on a black velvet chair, whereas the seat was a deep crimson plush. He laughed at his daughter’s description of the mistake he made, and declared he was only colour ignorant, and that she was the one who was colour blind. The examination showed that colour ignorant he was, but that the ignorance was due to complete red-blindness. For the seventy-four years he had lived he was unaware of his deficiency, suspecting it in others, and it was only an accidental circumstance which made him acquainted with the true state of his colour perception. Another elderly gentleman, in a high position in life, was also accidentally tested, and he proved to be completely green-blind. He, too, was quite unaware of his defect, and protested that, yachtsman as he was, he would never mistake a ship’s lights; but a very brief test showed his friends who were with him that his declaration had to be received with a certain amount of reservation. Others there are who certainly do know that some peculiarity exists in their sense of colour, and, foolish as it may appear to be—though, after all, it is quite consistent with a sensitive nature—they have tried to hide their defect from their fellow-creatures. Such examples, no doubt, some of my audience have met with, and experience tells me that they have just as much reluctance to pass an hour in my darkened room as they would have to occupy a police cell. In those few cases that have come voluntarily to me for examination, the peculiarity in colour sense was first brought to notice by the patient—if patient I may call him—failing to distinguish between cherries and the cherry leaves, or strawberries and the strawberry leaves. Such mistakes committed publicly are usually the source of unbounded merriment and curiosity to schoolboys when made by their schoolfellows, and I am bound to say that even persons of graver years are not unapt to be amused at what they consider to be a shortcoming in their fellow-creatures. To the student of colour vision the discovery of curious cases of colour deficiency is looked upon in a very different light—a good case of colour blindness, or still better one of monochromatic vision, is eagerly sought after, with the hope of submitting it to a rigid examination. When we look at the diagram ([Fig. 16]) we shall find why it is that the colour blind describe the spectrum as they do. Literally for those whose vision is dichromic, it is made up of two sensations alone, and the colours to which these sensations give rise are mixed throughout a large part of the spectrum, the pure unmixed sensations being at each end of the spectrum as they are in normal colour vision. The annexed diagram ([Fig. 18]) gives the curves for a red-blind person as made by observations under Clerk Maxwell’s directions. The standard colours here have been badly selected, for one of them stimulates the two sensations possessed.
Fig. 18.
An easy and instructive experiment can be made to give an idea of the kind of colour that these colour blind imagine as white, whether they be red-, green-, or violet-blind. (For those who have only monochromatic vision, as before stated, white is coloured with the one colour they possess.) Three slits are now in the spectrum, one near the extreme end of the red, another well in the violet, and the third in that part of the spectrum in which the green-blind see their neutral colour (see [page 66]). With the three colours issuing from these apertures a match is made with the white patch, and in this case the match is made as seen from a distant point, so that the resulting deductions may be true to the audience. If a colour-blind person be in this theatre, he will agree with me that the match is as correct to him as it is to myself and the rest of you. So far we could not distinguish his colour perception from the normal, but if he be red-blind, and the red slit be covered, he will still say that the match holds good, for, as a matter of fact, the red with which we helped to build up the white is non-existent to him. The white that he now sees is to us the greenish-blue patch which the mixed violet and green make. 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. We have, however, met with what is an apparent paradox. The green, coming through the now covered slit, he calls white, as he also does the purple. To impress this point more strongly upon you, I will place in front of the green slit a small prism which has an angle of about one and a-half degrees. This is just sufficient to throw the green colour on the neighbouring white surface. Here we have both the colours which the green-blind calls white side by side. If the brightness of each be the same, he would see no difference in them. Is it possible that on any theory this can be correct? To explain this apparent paradox, and without reference to the mathematical proof that white subtracted from white leaves white, we have only to look at our diagram ([Fig. 16]), and it is immediately apparent how it arises. The red and the blue curves cut at this point; and if we take away the green sensation entirely, the residue will be a mixture of the red and blue, which is the identical purple colour forming the patch.
If we are wishful to ascertain the colour that the violet-blind calls white, we have only to cover up the violet slit and a yellow is left behind as the result. I would have you remark that these colours which are seen as white would only be of the hues shown you, supposing the colour sensations were identical with those in normal vision. Whether this is the case we cannot absolutely say, and the only way in which this can be authoritatively settled is by examining some person who has normal colour vision in one eye and defective colour sense, not due to disease, in the other. One such person has been examined abroad, but in what way I am unable to say. It is recorded that he sees the red end of the spectrum as yellow with the eye that is defective. Another person I have heard of in England, but so far have not had the good fortune to get hold of him for examination. When I can lay my hands on him, he will be able to help to confirm or disprove what should be a general rather than a particular case.
So far I have only met with what appears to be one genuine case of violet blindness. It is very remarkable, on account of the eccentricity of the colour nomenclature. The only two colours which the subject saw were red and black. He named all greens and blues as black, the distinction between the two being that the former was “bright black” and the latter “dark black.” Yellow he called white, and a glance at [Fig. 16] will show that at this place in the spectrum the neutral point of a violet-blind should occur. By shifting the slit gradually into the green, he called it grey, instead of “bright black,” though it did not match the white patch when darkened. He called a green light a “bright black” light. We shall have to refer to this case when we are describing other investigations.