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colour depend, and that it is not a part of the object itself. The meaning of this will be best understood by an example. When a ray of light falls on the green grass, part of the ray is absorbed and part reflected, and the grass is only seen with the part that is reflected. The green we see consists of the original white light, deprived of a portion of its rays by absorption. It is, therefore, partial darkness, and not absolute light, consequently not a pure and absolute green, but only a residual group of the unabsorbed coloured rays. A poppy appears scarlet, as it absorbs all the colours of the rays except red, and hence its peculiar tint; but if it be looked at through green glass it will appear black : as the poppy only reflects the red ray, this is absorbed by the green glass. The red of the rose, the blue of the violet, the yellow of the jonquil, are due to their absorption of all the rays excepting the red, blue, and yellow. The pale-tinted rose, almost white, reflects nearly all the coloured rays. We can, therefore, easily perceive, without light, the face of nature would be that of a world in mourning; it is light that enlivens the scene, painting the exterior with a beauty, richness, delicacy, and harmony that man vainly attempts to rival. Colour is so dependent on light, that when artificially produced, as by candle or gas, from not being pure, many things appear of a different colour, as is well known by the lady who attempts to choose a ribbon or the artist who paints a picture by artificial light: a blue being mistaken for a green, and a green for a blue. On a moonlight night we cannot distinguish the colour of a chimney-pot; and were we to take a number of pieces of cloth, or different coloured papers, and examine them by the bright light of the moon, and write on the back of each the colour it appears, we should be astonished in daylight to see how much we had been deceived as to the true tint of each.
Assuming, therefore, that the sound eye can see perfectly well three simple colours-red, yellow, and blue—and that all the rest of the colours of the spectrum are mixtures of these with each other, let us now proceed to inquire what is the peculiar condition of sight in those persons who, being unable to distinguish certain rays, are, as we have already stated, colour-blind; but not necessarily owing to disease of the optic nerve or retina, but simply arising from inability to recognize those rays of light which consist of pure red.
Professor Maxwell, who has closely and philosophically investigated the subject, says :-" The mathematical expression of the difference between the colour-blind and ordinary vision is, that colour to the former is a function of two independent variables, but to an ordinary eye of three; and that the relation of the two kinds of vision is not arbitrary, but VOL. 11.—NO. VIII.
indicates the absence of determinate sensation, depending upon some undiscovered structure or organic arrangement, which forms one-third of the apparatus by which we receive sensations of colour.
“Suppose the absent structure to be that which is brought most into play when red light falls on our eyes, then to the colour-blind red light will be visible only so far as it affects the other two sensations, say of blue and green. It will, therefore, appear to them much less bright than to us, and will excite a sensation not distinguishable from that of a bluishgreen light.”
That is to say, the normal eye reduces its colour-sensations to three, and analyzes white light into three coloured elements, one of which is red; and that the colour-blind eye, on the other hand, reduces its colour-sensations to two, and analyzes white light into two elements, neither of which is red; for colour-blindness takes its character more from its nonrecognition of red than its positive recognition of yellow and violet. An essential distinction which can thus be drawn between perfect vision and colour-blindness has induced Sir J. Herschell to adopt the term dichromatic (cognizant only of two colours) to characterize the colour-blind.* We shall now examine how far the withdrawal of the red ray affects other colours. In the first place, all the light tints, as well as the dark tints, are liable to be mistaken for each other. The orange is no longer red and yellow, but dark yellow; the yellow is purer, the green distinct, the blue purer, and the indigo and violet no longer red and blue, but blue mixed with more or less black, 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 perfectly black. The red is generally seen as grey, or neutral tint; the orange as a dingy yellow; the blue as a dirty indigo, and the violet as a pale blue, mixed with black and grey.
In the “ Philosophical Magazine” for 1857 and 1862 will be found a series of experiments, instituted by Professor Maxwell, to test the accuracy of his own eyes in distinguishing between shades of colour; and his data may be followed by any one curious in the same field of inquiry. A large variety of all shades and tints of coloured wools may be used for the purpose. They should be placed in a mixed heap before the person, who must try to arrange and name them, beginning with the
* Dr. Wilson employs the term chromato-pseudopsis (false vision of colours), as it, he says, “very fairly expresses the general character of the affection, which more frequently shows itself as an insensibility to certain colours, than as a total inability to discern them.”
darkest, and putting those tints together that are most alike. Professor Maxwell adds, “The intelligent testimony of the colour-blind may supply a sure foundation for the theory of vision."
Many other curious and interesting points in connection with the philosophical part of our inquiry might be entered upon did the space at command permit us to do so; but enough has been said about light and colour to enable the reader to comprehend the more intricate part of the subject we are about to enter upon-namely, colour-blindness. As I have already said, the defect does not necessarily interfere with the integrity of the eye as an optical instrument. Indeed, in a case recorded by Dr. Wilson of a Mr. R- , an engraver, he counts himself not a sufferer, but a gainer by his colour-blindness. “Thus, an engraver has two negative colours to deal with-black and white. Now, when I look at a picture, I see it only in white and black, or light and shade; and any want of harmony in the colouring of a picture is immediately made manifest by a corresponding discord in the arrangement of the light and shade, or, as artists term it, the effect. I find, at times, many of my brother engravers in doubt how to translate certain colours of pictures which to me are matters of decided certainty and ease. Thus, to me it is valuable. I am totally unable to retain certain colours in my mind, nor able to give their names when shown to me a second time. Sometimes I can see some reds and greens by lamplight. A few years ago I ventured to buy some green baize; but unfortunately bought a very bright red, which was excessively painful to my eyes by lamplight, but agreeable enough by daylight. One of my brothers is equally defective, and my grandfather was very deficient in his knowledge of colours. My sight is natural, and rather powerful; for I am able to see very minute objects without assistance from glasses, and I can also see very distinctly with but little light. With regard to the rainbow, or solar spectrum, I can see clearly there are different shades of colour, but I am unable to say which is the red. The violet and yellow are very clear and distinct."
Those who have compared a coloured drawing or oil painting with an engraving of it will appreciate the nature of the difficulty which Mr. R- so easily surmounts. In heraldic engraving, for example, a system has long been followed of representing each colour by a separate set of marks. It comes, however, to be a very curious question whether this gentleman's version of a picture would satisfy one whose perception of colours was perfect. Professor Kelland and Dr. Wilson think it would not, as they have observed in the course of their inquiry that colour-blind persons arrange different shades of the same colour according to their intensity, in a series which did not satisfy their eyes; and further, that their arrangement of different colours according to their intensities seemed discordant to both these gentlemen.
The celebrated Dugald Stewart, and Dr. Darwin, the poet and botanist, could only by shape discover the difference between cherries and the leaves among which they grow. Dr. Dalton, the propounder of the atomic theory in chemistry, was not convinced he was colour-blind, until by accident observing the colour of the flower of the Geranium zonale by candlelight in the autumn of 1792. The flower was pink, but it appeared to him almost a sky-blue by day : in candlelight, however, it was astonishingly changed, not having then any blue in it, but being what he called red; forming a striking contrast to the blue. He also compared sealing-wax to one side of a laurel leaf, and a red wafer to the other, and his doctor's scarlet gown to the leaves of trees. “I have seen specimens," writes Dr. Dalton, “of crimson, claret, and mud which were very nearly alike. Crimson has a grave appearance, being the reverse of every showyor splendid colour. The colour of a florid complexion appears to me that of a dull, opaque, blackish blue upon a white ground. Diluted black ink upon white paper gives a colour much resembling that of a florid complexion. It has no resemblance to the colour of blood." From the care with which Dr. Dalton investigated his own defect, it has become popularly known as “Daltonism.” Nor was his case at all peculiar with regard to flowers, for the colour-blind are constantly found unable to distinguish the petals of the scarlet geranium from its leaves, the flowers of the wild poppy from the unripe corn amongst which it is growing. Moreover, those who thus mistake scarlet, regard green as a darkish colour, and confound it with drab.
The number of cases now upon record of persons afflicted in this way are very considerable ; though until within these late years it was supposed to be confined to a very few individuals. From the calculations of various authors, that one person out of every fifteen is colour-blind, and from the investigations of the late Dr. Wilson upon 1,154 persons at Edinburgh made in 1852-53, we gather that,
1 in 55 confounded red with green,
1 in 46 confounded blue with green ; hence, that one in nearly every eighteen had this imperfection. Professor Siebeck found five out of forty youths in the two upper classes in a school at Berlin colour-blind. Professor Prevost considers it occurs on an average in one out of twenty persons; and Wartmann, whose investigations almost exhaust the subject, thinks this estimate is not exaggerated. M. Lubeck rejects this conclusion as unsound, from the observations having been made in England and Germany, where blue is the prevailing colour of the eyes ; and it is a question with him whether it occurs so frequently in persons the irides-colour of whose eyes are black or hazel. In answer to this, it seems the great majority of cases examined by Wartmann had black irides.
This consideration, however, cannot be of much importance beyond the physiological correspondence observable with the ophthalmoscope between the colour of the iris and the fundus of the eye, by the relative determination of the pigmentum nigrum * in persons of different complexions. In adapting the eye to varying intensities of light, the pupil (iris) of course acts a principal part as to the amount of visual rays received, but its changes cannot have much effect upon the varying intensities of the vibrations to which the supplementary phenomena of colours are ascribed. It is the intensity rather than the character of the light that the iris controls, and which remains the same whatever sensation of colour is excited. It is different with regard to the influence which sex seems to exert, for on an analysis of upwards of two hundred cases, the proportion of males affected is no less than nine-tenths of the whole. Thus, it would appear that in this respect, the perfection of vision, the ladies have greatly the advantage over the gentlemen. There is, however, an interesting account given by M. Cumer of a family of thirteen females (extending through five generations), all of whom were colour-blind. On the other hand, Dr. Bronner, of Paris, relates the case of a learned chemist, a German, whose two daughters were free from their father's defect. The children of the eldest one were likewise unaffected, whereas three sons of the youngest were all colour-blind. A grandson, also, the son of one of these latter, inherited the defect. In the “ American Journal of Medical Science,” 1854, another similar case is reported, where seventeen descendants, chiefly males, of the maternal grandfather all inherited colour-blindness.
The two elder sons out of a family of four suffer from this defect. The second son, now an eminent sculptor, early in life exhibited great taste in drawing and painting, but after some few years of study was obliged to relinquish the art, in
* I make no apology for introducing technical terms into this paper, since the anatomy of the eye is so well explained in a paper which appeared in the January part of the present year. See page 220,