Colour Blindness

Colour Blindness, an inability to distinguish between certain colours exists in some persons. John Dalton, the distinguished chemist, was afflicted in this way, and he, by a description of the differences which he found to exist between his colour judgments and those of ordinary persons, first directed the attention of scientific men to the subject. For this reason colour blindness is sometimes termed Daltonism. The examination of a large number of individuals has shown that some form of colour blindness is present in about 4 per cent. of the male population of civilised countries. Curiously enough, such a defect is very much more rarely met with in women. Attempts have been made to reconcile the phenomena of colour blindness with one or other of the theories of colour vision. There are alike difficulties, however, in applying either the Young-Helmholtz hypothesis or the theory of Hering to the actual mistakes observable in colour-blind persons, and the question must be regarded as still sub judice. The most commonly noticeable defect in the subjects of colour blindness is an inability to distinguish between red and green, and as these are the two colours most employed for signalling purposes, the practical importance of instituting some adequate test of normal colour vision is very great. The matter is by no means so simple as would at first sight appear. A colour-blind person will often succeed in correctly naming a series of colours, and in many instances he may be quite unaware that his colour perceptions differ in any way from those of the majority of mankind; indeed, in some of the more slightly marked cases of colour blindness it is only by a most carefully conducted examination that the abnormality can be demonstrated. On the other hand, it is a familiar occurrence for an uneducated person to be hopelessly at fault in naming colours, quite apart from any actual defect of the nature of colour blindness. A very useful test was devised by Holmgren, and the sets of coloured skeins of wool introduced by him are largely employed by ophthalmic surgeons in this country. Briefly, the examination is conducted as follows: A test skein is handed to the patient, and he is requested to match it from a number of other skeins. A normal eye has no difficulty in separating the wools of the same colour as the test skein from the rest, which are known as the "confusion colours." A colour-blind person, on the other hand, matches one or more of the confusion skeins with the test skein. Unfortunately, the methods adopted for the exclusion of colour-blind employes by railway companies and ship-owners are very rough and ready; it is only within very recent times, however, that the desirability of conducting any examination at all has been recognised, and doubtless more and more attention will be devoted to the matter now that its importance is so well established.