Colour Blindness.
If there is one infirmity or defect of those five senses with which we are most of us blest, which more than any other attracts sympathy and claims compassionate consideration, it is blindness—an inability to know what is beautiful in form or in colour, to appreciate light, or to recognize and comprehend the varying features of our fellow-men—a perpetual darkness in the midst of a world of light—a total exclusion from the readiest, pleasantest, and most available means of acquiring ideas.
And yet who would suppose that there exists, and is tolerably common, a partial blindness, which has hardly been described as a defect for more than half a century, and of which it may be said even now that most of those who suffer from it are not only themselves ignorant of the fact, but that those about them can hardly be induced to believe it. The unhappy victims of this partial blindness (which is real and physical, not moral) are at great pains in learning what to them are minute distinctions of tint, although to the rest of the world they are differences of colour of the most marked kind, and, after all, they only obtain the credit of unusual stupidity or careless inattention in reward for their exertions and in sympathy for their visual defect. We allude to a peculiarity of vision which first attracted notice in the case of the celebrated propounder of the atomic theory in chemistry, the late Dr. Dalton, of Manchester, who on endeavouring to find some object to compare in colour with his scarlet robe of doctor of laws, when at Cambridge, could hit on nothing which better agreed with it than the foliage of the adjacent trees, and who to match his drab coat—for our learned doctor was of the Society of Friends—might possibly have selected crimson continuations as the quietest and nearest match the pattern-book of his tailor exhibited.
An explanation of this curious defect will be worth listening to, the more so as one of our most eminent philosophers, Sir John Herschel, has recently made a few remarks on the subject, directing attention at the same time to other little known but not unimportant phenomena of colour, which bear upon and help to explain it.
It is known that white light consists of the admixture of coloured rays in certain proportions, and that the beautiful prismatic colours seen in the rainbow are produced by the different degree in which the various rays of colour are bent when passing from one transparent substance into another of different density. Thus, when a small group of colour-rays, forming a single pencil or beam of white sunlight, passes into and through the atmosphere during a partial shower, and falls on a drop of rain, it is first bent aside on entering the drop, then reflected from the inside surface at the back of the drop, and ultimately emerges in an opposite direction to its original one. During these changes, however, although all the colour-rays forming the white pencil have been bent, each has been bent at a different angle—the red most, and the blue least. When therefore they come out of the drop, the red rays are quite separated from the blue, and when the beam reaches its destination, the various colours enter the eye separately, forming a line of variously coloured light, the upper part red and the lower part blue, instead of a mere point of white light, as the ray would have appeared if seen before it entered the drop. The eye naturally refers each part of the ray to the place from whence it appears to come, and thus, with a number of drops falling and the sun not obscured, a rainbow is seen, which represents part of a number of concentric circular lines of colour, the outermost of which is red, the innermost violet, and the intermediate ones we respectively name orange, yellow, green, blue, and indigo.
It has also been found by careful experiment, that these are not all pure colours, most of them being mixtures of some few that are really primitive and pure, and necessarily belong to solar light. It is these mixed in due proportion which make up ordinary white light, which is the only kind seen when the sun’s rays have not undergone this sort of decomposition or separation into elements. The actual primitive colours are generally supposed to be red, yellow, and blue, and much theoretical as well as practical discussion has arisen as to how these require to be mixed, what proportion they bear to each other in their power of impressing the human eye, and many other matters for which we must refer to Mr. Field, Mr. Owen Jones, and others, who have studied the subject and applied it.
In a general way it is found convenient to remember, or rather to assume, that three parts of red, five parts of yellow, and eight parts of blue form together white, and, therefore, that the pencil of white light contains three rays of red, five of yellow, and eight of blue. To produce the other prismatic colours, we must mix red with a little yellow to form orange; yellow with some blue to form green; much blue with a little red to form indigo, and a little blue with some red to form violet. In performing experiments on colour it is convenient, instead of a drop of water, to substitute a prism of glass in decomposing the rays of light. We may thus produce at will a convenient image, called a prismatic spectrum, which, when thrown on a wall, is a broad band of coloured lights, having all the tints of the rainbow in the same order. Looking at this image, the red is at the top and the violet at the bottom, and it may be asked, How does the red get amongst the blue to form violet, if the red rays are bent up to the top of the spectrum? The answer is, that a quantity of white light not decomposed, and a part of all the colour rays, reach all parts of the spectrum, however carefully it is sheltered, but that so many more red rays get to the top, so many more of the yellow to the middle, and so many more blue to where that colour appears most brilliant, that these are seen nearly pure, whilst where the red and yellow or yellow and blue mix they produce distinct kinds of colour, and where the blue at the bottom is faint, and some of those red rays fall that do not reach the red part of the spectrum, the violet is produced. In point of fact, therefore, all the colours of the spectrum, as seen, are mixtures of pure colour with white light, while all but red are mixtures of other pure colours with some red and some yellow as well as white. Primitive and pure colours, therefore, are not obtained in the spectrum, and a question has arisen as to which really deserve to be called pure, Dr. Young upholding green against yellow, and even regarding violet as primitive, and blue a mixed colour. A consideration of the results of this theory would lead us farther than is necessary for the purpose we have now in view.
We also find philosophers now-a-days calmly discussing a question which most people considered settled very long ago, namely, whether blue and yellow together really make green.
It is of no use for the artist to lift up his eyes with astonishment at any one being so insane as to question so generally admitted a statement. In vain does he point to his pictures, in which his greens have been actually so produced. The strict photologist at once puts him down, by informing him that he knows little or nothing of the real state of the case: his (the artist’s) colours are negative, or hues of more or less complete darkness; whereas in nature, the colour question is to be decided by positive colours, or hues in which all the light used is of one kind. The meaning of this will be best understood by an example: When a ray of white light falls on a green leaf, part of the ray is absorbed and part reflected, and the object is therefore only seen with the part that is reflected. That which is absorbed consists of some of each of the colour rays, and the resulting reflected light is nothing more than a mixture of what remains after this partial absorption. The green we see consists of the original white light deprived of a portion of its rays. It is not a pure and absolute green, but only a residual group of coloured rays, and thus in so far the green colour is negative, or consists of rays not absorbed. It is therefore partial darkness, and not absolute light. If, however, on the other hand, a ray of white light is passed through a transparent medium (e. g. some chemical salt) which has the property of entirely absorbing all but one or more of the colour rays, and no part of the remainder, then all the light that passes through this medium is of the one colour, or a mixture of the several colours that pass: and if such light is thrown on a white ground, the reflected colour will be positive, and not negative, and is far purer as well as brighter than the colour obtained in the other way. It has been found by actual experiment, that when positive blue, thus obtained, is thrown on positive yellow, the resulting reflected colour bears no resemblance to green. Sir John Herschel considers, that whether green is a primitive colour—in other words, whether we really have three or four primitive colours—remains yet an open question.
It was necessary to explain these matters about colour before directly referring to the subject of this paper, namely, blindness to certain colour rays. It should also be clearly understood that the persons subject to this peculiar condition of vision have not necessarily any mechanical or optical defect in the eye as an optical instrument, which may be strong or weak, long-sighted or short-sighted, quite independently of it. Colour blindness does not in any way interfere with the ordinary requirements of vision, nor is there the smallest reason to imagine that it can get worse by neglect, or admit of any improvement by education or treatment.
Assuming that persons of ordinary vision see three simple colours, red, yellow, and blue, and that all the rest of the colours are mixtures of these with each other and with white light, let us try to picture to ourselves what must be the visual condition of a person who is unable to recognize certain rays; and as it appears that there is but one kind of colour-blindness known, we will assume that the person is unable to recognize those rays of white light which consist of pure red and nothing else. In other words, let us investigate the sensations of a person blind so far only as pure red is concerned.
All visible objects either reflect the same kind of light as that which falls on them, absorbing part and reflecting the rest, or else they absorb more of some colour rays than others, and reflect only a negative tint, made up of a mixture of all the colour-rays not absorbed. To a colour-blind person, the mixed light, as it proceeds from the sun, is probably white, as seen by those having perfect vision; for, as we have explained already, positive blue and yellow (the colour rays when red is excluded) do not make green, and the absence of the red ray is likely to produce only a slight darkening effect. So far, then, there is no difference. But how must it be with regard to colour.
Bearing in mind what has been said above, it is evident that in withdrawing the red rays from the spectrum, we affect all the colours. The orange is no longer red and yellow, but darkened yellow; the yellow is purer, the green is quite distinct, the blue purer, and the indigo and violet no longer red and blue, but blue mingled with more or less of darkness, the violet being the darkest, as containing least blue in proportion to red, while the red part itself, though not seen as a colour, is not absolutely black, inasmuch as its part of the spectrum is faintly coloured with the few mixed rays of blue and yellow and white that escape from their proper place. The red then ought to be seen as a gray neutral tint, the orange a dingy yellow, the indigo a dirty indigo, and the violet a sickly, disagreeable tint of pale blue, darkened considerably with black and gray.
Next let us take the case of an intelligent person affected with colour blindness, but who is not yet aware of the fact. He has been taught from childhood that certain shades, some darker and some brighter, but all of neutral tint, and not really presenting to him colour at all, are to be called by various names—scarlet, crimson, pale red, dark red, bright red, dark green, dark purple, brown, and others. With all these he can only associate an idea of gray; nor can he possibly know that any one else sees more than he does. Having been taught the names they are called by, he remembers the names, with more or less accuracy, and thus passes muster. There is a real difference of tint, because each of these colours consists of more or less blue, yellow, and white, mixed with the red; and our friend is enabled to recognize and name them, more or less correctly, according to his acuteness of perception and accuracy of memory.
If we desire to experiment on such a person, we must ask no names whatever, but simply place before him a number of similar objects differently coloured. Taking, for example, skeins of coloured wools, let us select a complete series of shades of tint, from red, through yellow, green, and blue, to violet, and request him to arrange them as well as he is able, placing the darkest shades first, and putting those tints together that are most like each other. It is curious then to watch the progress of the arrangement. In a case lately tried by the writer of this article, the colour-blind person first threw aside at once a particular shade of pale green as undoubted white, and then several dark blues, dark reds, dark greens, and browns, were put together as black. The yellows and pure blues were placed correctly, as far as name was concerned, by arranging several shades in order of brightness—but the order was very different from that which another person would have selected. The greens were grouped, some with yellows, and some with blues.
The colours in this experiment were all negative and impure, but we may also obtain something like the same result with positive colour, transmitted by the aid of polarized light through plates of mica. In a case of this kind described by Sir J. Herschel, the only colours seen were blue and yellow, while pale pinks and greens were regarded as cloudy white, fine pink as very pale blue, and crimson as blue; white red, ruddy pink, and brick red were all yellows, and fine pink blue, with much yellow. Dark shades of red, blue, or brown, were considered as merely dark, no colour being recognized.
The account of Dr. Dalton’s own peculiarity of vision by himself, offers considerable interest. He says, speaking of flowers: “With respect to colours that were white, yellow, or green, I readily assented to the appropriate term; blue, purple, pink, and crimson appeared rather less distinguishable, being, according to my idea, all referable to blue. I have often seriously asked a person whether a flower was blue or pink, but was generally considered to be in jest.” He goes on further to say, as the result of his experience: “1st. In the solar spectrum three colours appear, yellow, blue, and purple. The two former make a contrast; the two latter seem to differ more in degree than in kind. 2nd. Pink appears by daylight to be sky-blue a little faded; by candlelight it assumes an orange or yellowish appearance, which forms a strong contrast to blue. 3rd. Crimson appears muddy blue by day, and crimson woollen yarn is much the same as dark blue. 4th. Red and scarlet have a more vivid and flaming appearance by candlelight than by daylight” (owing probably to the quantity of yellow light thrown upon them).
As anecdotes concerning this curious defect of colour vision, we may quote also the following: “All crimsons appeared to me (Dr. Dalton) to be chiefly of dark blue, but many of them have a strong tinge of dark brown. I have seen specimens of crimson claret and mud which were very nearly alike. Crimson has a grave appearance, being the reverse of every showy or splendid colour.” Again: “The colour of a florid complexion appears to me that of a dull, opaque, blackish blue upon a white ground. Dilute black ink upon white paper gives a colour much resembling that of a florid complexion. It has no resemblance to the colour of blood.” We have a detailed account of the case of a young Swiss, who did not perceive any great difference between the colour of the leaf and that of the ripe fruit of the cherry, and who confounded the colour of a sea-green paper with the scarlet of a riband placed close to it. The flower of the rose seemed to him greenish blue, and the ash gray colour of quick-lime light green. On a very careful comparison of polarized light by the same individual, the blue, white, and yellow were seen correctly, but the purple, lilac, and brown were confounded with red and blue. There was in this case a remarkable difference noticed according to the nature and quantity of light employed; and as the lad seemed a remarkably favourable example of the defect, the following curious experiment was tried. A human head was painted, and shown to the colour-blind person, the hair and eyebrows being white, the flesh brownish, the lips and cheeks green. When asked what he thought of this head? the reply was, that it appeared natural, but that the hair was covered with a nearly white cap, and the carnation of the cheeks was that of a person heated by a long walk.
There is an interesting account in the Philosophical Transactions for 1859 (p. 325), which well illustrates the ideas entertained by persons in this condition with regard to their own state. The author, Mr. W. Pole, a well-known civil engineer, thus describes his case:—“I was about eight years old when the mistaking of a piece of red cloth for a green leaf betrayed the existence of some peculiarity in my ideas of colour; and as I grew older, continued errors of a similar kind led my friends to suspect that my eyesight was defective; but I myself could not comprehend this, insisting that I saw colours clearly enough, and only mistook their names.
“I was articled to a civil engineer, and had to go through many years’ practice in making drawings of the kind connected with this profession. These are frequently coloured, and I recollect often being obliged to ask in copying a drawing what colours I ought to use; but these difficulties left no permanent impression, and up to a mature age I had no suspicion that my vision was different from that of other people. I frequently made mistakes, and noticed many circumstances in regard to colours, which temporarily perplexed me. I recollect, in particular, having wondered why the beautiful rose light of sunset on the Alps, which threw my friends into raptures, seemed all a delusion to me. I still, however, adhered to my first opinion, that I was only at fault in regard to the names of colours, and not as to the ideas of them; and this opinion was strengthened by observing that the persons who were attempting to point out my mistakes, often disputed among themselves as to what certain hues of colour ought to be called.” Mr. Pole adds that he was nearly thirty years of age when a glaring blunder obliged him to investigate his case closely, and led to the conclusion that he was really colour-blind.
All colour-blind persons do not seem to make exactly the same mistakes, or see colours in the same way; and there are, no doubt, many minor defects in appreciating, remembering, or comparing colours which are sufficiently common, and which may be superadded to the true defect—that of the optic nerve being insensible to the stimulus of pure red light. It has been asserted by Dr. Wilson, the author of an elaborate work on the subject, that as large a proportion as one person in every eighteen is colour-blind in some marked degree, and that one in every fifty-five confounds red with green. Certainly the number is large, for every inquiry brings out several cases; but, as Sir John Herschel remarks, were the average anything like this, it seems inconceivable that the existence of the defect should not be one of vulgar notoriety, or that it should strike almost all uneducated persons, when told of it, as something approaching to absurdity. He also remarks, that if one soldier out of every fifty-five was unable to distinguish a scarlet coat from green grass, the result would involve grave inconveniences that must have attracted notice. Perhaps the fact that a difference of tint is recognized, although the eye of the colour-blind person does not appreciate any difference of colour, when red, green, and other colours are compared together, and that every one is educated to call certain things by certain names, whether he understands the true meaning of the name or not, may help to explain both the slowness of the defective sight to discover its own peculiarity, and the unwillingness of the person of ordinary vision to admit that his neighbour really does not see as red what he agrees to call red.
There is, however, another consideration that this curious subject leads to. It is known that out of every 10,000 rays issuing from the sun, and penetrating space at the calculated rate of 200,000 miles in each second of time, about one-fifth part is altogether lost and absorbed in passing through the atmosphere, and never reaches the outer envelope of the human eye. It is also known that of the rays that proceed from the sun, some produce light, some heat, and some a peculiar kind of chemical action to which the marvels of photography are due. Of these only the light rays are appreciated specially by the eye, although the others are certainly quite as important in preserving life and carrying on the business of the world. Who can tell whether, in addition to the rays of coloured light that together form a beam of white light, four-fifths of which only pass through the atmosphere, there may not have emanated from the sun other rays altogether absorbed and lost? or whether in entering the human eye, or being received on the retina at the back of the eye, or made sensitive by the optic nerve, there may not have been losses and absorptions sufficient to shut out from us, who enjoy what we call perfect vision, some other sources of information. How, in a word, do we who see clearly only three or four colours, and their various combinations, together with their combined white light—how do we know that to beings otherwise organized, the heat, or chemical rays, or others we are not aware of, may not give distinct optical impressions? We may meet one person whose sense of hearing is sufficiently acute to enable him to hear plainly the shrill night-cry of the bat, often totally inaudible, while his friend and daily companion cannot perhaps distinguish the noise of the grasshopper, or the croaking of frogs, and yet neither of these differs sufficiently from the generality of mankind to attract attention, and both may pass through life without finding out their differences in organization, or knowing that the sense of hearing of either is peculiar. So undoubtedly it is with light. There may be some endowed with visual powers extraordinarily acute, seeing clearly what is generally altogether invisible; and this may have reference to light generally, or to any of the various parts of which a complete sunbeam is composed. Such persons may habitually see what few others ever see, and yet be altogether unaware of their powers, as the rest of the world would be of their own deficiency.
The case of the colour-blind person is the converse. He sees, it is true, no green in the fields, or on the trees, no shade of pink mantling in the countenance, no brilliant scarlet in the geranium flower, but still he talks of these things as if he saw them, and he believes he does see them, until by a long process of investigation he finds out that the idea he receives from them is very different from that received by his fellows. He often, however, lives on for years, and many have certainly lived out their lives without guessing at their deficiency.
These results of physical defects of certain kinds remaining totally unknown, either to the subject of them or his friends, even when all are educated and intelligent, are certainly very curious; but it will readily be seen that they are inevitable in the present development of our faculties. In almost everything, whether moral or intellectual, we measure our fellows by our own standard. He whose faculties are powerful, and whose intellect is clear, looks over the cloud that hovers over lower natures, and wonders why they, too, will not see truth and right as he sees them. Those, on the other hand, who dwell below among the mists of error and the trammels of prejudice, will not believe that their neighbour, intellectually loftier, sees clearly over the fog and malaria of their daily atmosphere.
In taking leave of the question of colour blindness, it should be mentioned that hitherto no case has been recorded in which this defect extends to any other ray than the red.
There seems no reason for this, and possibly, if they were looked for, cases might be found in which the insensibility of the optic nerve had reference to the blue instead of the red ray—the least instead of the most refrangible part of the beam of light. It would also be well worth the trial if those who have any reason to suppose that they enjoy a superiority of vision would determine by actual experiment the extent of their unusual powers, and learn whether they refer to an optical appreciation of the chemical or heat rays, or show any modification of the solar spectrum by enlargement or otherwise.
Lastly, it would be well, when children show an unusual difficulty in describing colours, to try by some such experiments as those here related whether any defect of colour blindness exists or not. It would clearly be undesirable that such children as have this defect should waste time in learning accomplishments or professions which they must always be unable to practise. They, their parents and teachers, may thus be saved some of that disappointment which is always experienced when presumed tastes and talents are cultivated or forced contrary to the natural powers of the individual. It must clearly be hopeless to endeavour to obtain good taste in colours, when most of the colours themselves are not seen at all, or are so recognized as to present appearances altogether different from those seen by the rest of the world.