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.