COLOURS, Incapacity of distinguishing. This peculiarity of vision was till lately supposed to be a very rare phenomenon. It appears first to have attracted attention in England, and some imperfectly described cases were recorded in the Phil. Trans. in the latter half of the preceding century. These examples were supposed to be unique, till the great chemist John Dalton announced himself in 1794 as exhibiting the peculiarity under notice, and described the cases of more than twenty persons similarly circumstanced. Later researches in England, France, Switzerland, Germany, Belgium, and America, have shown that this condition of vision is far from uncommon. Various names have been proposed for it, such as Daltonism from Dalton (an objectionable appellation which English writers all decline to use), achromatopsy, dyschromatopsy, chromatopseudopsy, &c. The most convenient English name is that of colour-blindness, suggested by Sir David Brewster, which, though not strictly accurate, is sufficiently distinctive to warrant its use even as a scientific term.
Colour-blindness is of two kinds. The one which is exceedingly rare is characterized by an inability to perceive any colour properly so called, the eye being only conscious of black and white, or rather of a very dark, and a very light gray. Living examples of this extreme colour-blindness have been recently described by English and foreign writers.
The other kind of colour-blindness, which is very common, shows itself in an inability to distinguish certain colours from each other, and to distinguish the very light tints of all colours from white, and the very dark shades from black. Those who err thus generally see in the rainbow only two colours, blue and yellow, but in the prismatic solar spectrum, looked at from a short distance, several additional coloured bands are distinguished.
The colours most liable to be confounded with each other are, 1. Red full and bright, including crimson, scarlet and the redder shades of orange, with full and bright green. 2. Brown, including dull red, with dark or dull green. 3. Purple, including pink, crimson, lilac, lavender, and other mixtures of red and blue throughout nearly all their shades, with blue. 4. Green with blue, even by daylight—the most common probably of all the forms of colour-blindness. 5. Red (including crimson and scarlet), with black. 6. Light tints of all colours, such as straw colour, and the equally pale shades of pink, blue, green, &c., with white. 7. Dark shades of all colours, such as claret, purple, olive-blue, with black. Popular interest has been chiefly excited by the mistakes of colour-blind persons between red and green; but in a scientific point of view the most singular mistake is that of full red for black, inasmuch as it implies that the eye thus deceived not only does not see red as red, but does not see it as any colour, or as in any way visible at all. Red light is thus equivalent to no light, and does not make even a luminous impression on the retina. The colour best distinguished by the colour-blind is yellow, and other colours are rarely mistaken for it. Blue, when pure and bright, is also perfectly distinguished, but purples and greens are often mistaken for blue.
Colour-blindness is greatly more common among males than females, but the relative proportion of cases in the two sexes has not been ascertained. Among males there is reason to believe that 1 in 20 is colour-blind in some degree; and that 1 in 50 is colour-blind to the extent of mistaking for each other the colours in the preceding list.
Colour-blindness, when marked, generally shows itself in more than one member of the same family, and has been traced from parent to child, through as many as five generations. It seldom, however, includes all the members of a family, and the whole of the females of a generation often escape. The eyes of the colour-blind present in general no peculiarity in appearance or internal structure, and the power of vision in reference to everything but colour is per-
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fectly normal, and often appears even superior to that of those who do not mistake colours.
The cause of colour-blindness is not known, but the great majority of authors on the subject are agreed in referring it to some peculiarity of structure in the retina or brain. It appears to be quite incurable, but some assistance in distinguishing colours is derived from the substitution of artificial light for daylight; and yellow spectacles have proved of some service by day.
The subject is of practical importance from the great extent to which coloured signals are employed on railways and at sea; but especially on the former, where the colours preferred, viz., red and green, are among the greatest stumbling blocks of the colour-blind.
The fullest account of this peculiarity of vision will be found in the papers of Professor E. Wartmann of Geneva, the first of which was published in French in 1840, and translated into English in 1846, in Taylor's Scientific Memoirs for that year; the second entitled Deuxième Mémoire sur le Daltonisme, &c., is published in the Mémoires lus à la Société de Physique de Genève, 1849. Some account of it is given in Mr W. Cooper's article Vision in the Cyclopædia of Anatomy. The latest English researches on the subject are contained in a series of papers by Dr George Wilson in the Edinburgh Monthly Journal of Medical Science for 1853-54.