Colour-blindness, a term introduced by Sir David Brewster to denominate a defect of vision owing to which some persons are unable to distinguish certain colours correctly. It is also called Achromatopsia (Gr.) and Daltonism, from Dalton the chemist, who suffered from the defect, and who gave the first detailed description of it (1794). Of this defect there are several degrees, classified as follows (by Holmgren of Upsala): (1) Total colour-blindness, where there is no perception of colours as such, but only of gradations of light and shade; (2) Complete partial colour-blindness, where some bright colours, different in different cases, are confused with each other, though other colours are correctly perceived; (3) Incomplete partial colour-blindness, where bright colours are recognised, but more delicate shades are confused. The first form is rare, and generally, perhaps always, associated with other defects in the eyes; the third is probably common, though not of great importance; to the second attention will mainly be directed here.
With regard to the classification of the cases of complete partial colour-blindness authorities are not agreed; the important practical point is that in the vast majority red and green are the colours confused. Some confuse a bright red with a green that appears to a normal eye a much lighter colour; some with a green that appears darker. But if suitable tests be applied it will be found that they do not distinguish red and green as such. Experience, however, and observation of the different apparent brightness of ordinary reds and greens, enables them to distinguish between them in most cases with wonderful accuracy, so that they may remain unconscious of their defect till some striking mistake, or the application of a systematic test, reveals it. Cases of colour-blindness for yellow and blue, if they occur at all, are extremely rare. Experience proves that this defect is generally hereditary, and is quite incurable. The eyes may be, and usually are, perfect in every other respect; no difference has been detected in their structure, either during life or after death; so the cause of their defective perception remains absolutely unknown.
Numerous careful and extensive researches both in various countries of Europe and in the United States have shown that this defect is present in about 4 per cent. of the males (or one in twenty-five), and less than 0·5 per cent. of the females in those countries. Now red and green are the very colours which are most largely used for the purpose of signalling both at sea and on railways; and it must be obvious that most disastrous results may follow if the person whose duty it is to distinguish them from each other is unable to do so. It is then an important practical question how the defect may best be recognised. Authorities on the subject are agreed that any test which requires the naming of colours is unsatisfactory, and that for two reasons. First, a person may perceive colours correctly, but may make mistakes through imperfect knowledge of their names; second, a person may be colour-blind, and yet by his perceptions of different brightness in the tests, may name the colours correctly. The most perfect test yet devised is known by the name of its introducer, Holmgren of Upsala, and consists in a number of skeins of wool of different shades. One of these is placed before the person to be tested, generally in the first instance a pale green, and he is asked to select from the remainder those which most resemble it. If colour-blind, he is sure to pick out some of the 'confusion colours,' pale-grays, buffs, &c., to match the green; and further similar tests may then be applied to determine more precisely his defect. Within recent years the importance of this defect has been recognised by the railway companies and the Board of Trade in Britain; though the tests applied are not in either case so satisfactory as is to be desired, as they consist in requiring the candidate to name coloured cards, lights, &c. Moreover, the Board of Trade examination is required only of men applying for certificates as mates or masters of vessels; there is no compulsory examination of ordinary seamen, though one of their most important duties is to keep a lookout at night for signal-lights, many of which are red and green. This arrangement is also productive of much hardship to those unaware of their defect, who have spent valuable years in the drudgery of preparation for a calling to which colour-blindness should be an absolute disqualification, and only find out their unfitness when they should begin to reap the reward of their labour. Further, in certain cases colour-blindness does not prevent the granting of a certificate by the Board of Trade; the fact that its holder is colour-blind is indorsed upon it, but he is permitted to act as master or mate notwithstanding, if he can find a shipowner to employ him.
There are difficulties in the way of substituting any other colours than red and green for signals. Blue and yellow are the only others sufficiently definite and contrasted for the purpose. But blue is much more quickly lost in passing through the air than other colours; and yellow is just the colour to which haze or distance reduces white.
Colour-blindness more or less complete may also occur as the result of disease of the eyes, particularly atrophy of the optic nerve and excessive use of tobacco. See EYE (Vol. IV. p. 513), and AMAUROSIS. In the former, the distance from the direct line of sight at which colours can be recognised is diminished; in the latter, the colour of a small disc is not recognised when it is in the direct line of sight, though it may still be perceived at a little distance from it. As in the latter case the perception of red and green is specially affected, its presence in engine-drivers, &c. may cause even greater risk than the congenital form of colour-blindness. For further information, see the Brit. Assoc. Report (1886); Roy. Soc. Report (1892); and works on colour-blindness and colour-vision by Jay Jeffries (Boston, U.S.), Eldridge Green (1891), and Abney (1895); and the writings of
Sir David Brewster, George Wilson, Clerk-Maxwell, &c.