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.