COLOUR-BLINDNESS, a term applied to a condition of vision in which the number of colours which can be differentiated is markedly less than normal. Many different types have been noted; Schjelderup recognizes at least 18 which need explanation by any theory of colour vision; and there are evidences of an in definite variety of gradations from full trichromatic vision to total colour-blindness in which all things appear in shades of grey only. However, the data available at present indicates the follow ing types as of principal importance : (1) anomalous trichroma tism, involving a minimum of departure from the normal; (2) dichromatism, in which all colours can be matched by mixtures of two, instead of requiring three, elementary stimuli; (3) mono chromatism, in which there is no discrimination of wave-length compositions, as such. Colour-blindness may rest either upon con genital and non-remediable conditions, or it may be a consequence of disease or injury to the optic nervous system. In the former case, the characteristics are biometrically more clear-cut than in the latter.
Colour-blindness of the first two types listed above is diffi cult to detect under ordinary conditions of life, because of the skill with which the affected individual utilizes brightness cues in judging the colours of objects. However, when presented with appropriately selected pigments, or with spectral stimuli, he may make egregious errors or show an easily measurable departure from normal. When an anomalous trichromat is required to match a spectral yellow with a mixture of spectral red and green, he may require either more red or more green than the normal, corres ponding to the protanomalous and deuteranomalous conditions, re spectively. For the protanomalous, reds are reduced in brightness as well as colour value, and the anomalous trichromats, in general, show weaknesses of the colour sense in the form of high thresholds, easy fatigability, increased contrast effects, and the like. It seems probable that anomalous trichromatism merely constitutes an ex treme variant of normal trichromatic vision, which naturally differs in its exact characteristics from one individual to another.
Dichromatism represents the most common form of colour blindness, being a characteristic of about 4% of males and about one-tenth this proportion of females. It shows the properties of a sex-linked character in the Mendelian scheme of heredity. These statements apply primarily to the two forms which are known as protanopia and deuteranopia, respectively. Tritanopia, a rarer dichromatic condition, is associated with pathological rather than with hereditary defects. Both of the first two forms are character ized by an inability to distinguish between reds and greens, when these colours are adjusted to the same brightness, for a given individual. In both types, a narrow region of the spectrum, hav ing a wave-length lying somewhere between 490 and 500 µµ, matches a white or grey, although for the normal observer this region is a slightly bluish green.
The brightness of the spectrum at the long-wave (red) end is very markedly reduced in the case of the protanope, yielding a distorted visibility curve, as compared with the normal. The deuteranope has a curve which is approximately the same as that of the normal trichromat. It is a general rule that colour matches or equations which are established by the latter are valid for the former, a fact which is taken to indicate that dichromatism is derived from trichromatism by the simple dropping out of a colour component. Tritanopia, or "blue-blindness," involves a confusion between blue and yellow, with a neutral band in the yellow region of the spectrum. It is associated with detachment of the retina. Data from cases of monocular colour-blindness indi cate that the colour sensations of the protanopes and deuteranopes are confined to yellow, grey and blue. The tritanopes presumably see the spectrum in terms of red, grey, and green.
Monochromatism is a condition in which all objects or stimuli appear to be of the same colour—various shades of grey. In practically all cases, there is an accompanying condition of day blindness or photophobia, confirming the view that monochromat ism is referable to complete functional loss of the retinal cone sys tem, which is responsible for chromatic discrimination in general. Vision is thus restricted to the rod system, which is achromatic even in the normal individual.
For a more detailed discussion of the facts and theories of colour blindness, see J. H. Parsons An Introduction to the Study of Colour Vision, Part II. (1924)• (L. T. T.)