Monochromatism*

Abstract

MONOCHROMATISM is a type of colour-vision deficiency in which the observer, a monochromat, can match any colour by any other colour, merely by the adjustment of their intensities. It is a very rare form of colour-vision deficiency, its frequency of occurrence in the male population being stated to be so low as 0·0003 per cent¹, as compared with 6 per cent for anomalous trichromatism and 2 per cent for dichromatism. Of this rather tentative 0·0003 per cent, nearly all the recorded cases are monochromats by virtue of their having no cone-vision. Such persons, whose defect is usually described as cone-blindness, rely solely on the functioning of the rods in the retina, and suffer from other ophthalmological defects such as low visual acuity and nystagmus. As might be expected of persons using a mechanism which, in normal persons, only functions when the eyes are dark-adapted, they suffer from photophobia or fear of the light. The luminosity curve for this type of monochromat is shown (Curve A) in the accompanying graph². It is very similar to the scotopic luminosity curve for the normal eye. Whether such persons may be classed as colour-blind in. the accepted sense of the word is a fine point; if they are accepted as such, then there is no logical reason why totally blind persons should not also be classed in the same way.