The disability in so-called red-green blindness: an account based on many years of self-observation.

THIS is an attempt to describe the disability in red-green blindness. It is not an account of the physiology of normal colour vision, nor of the abnormality of the cone cells of the retina in red-green blindness, nor of the genetics of the trait. It does not consider total colour blindness, or colour agnosia, or defects in colour memory. It must be remembered however that the trait is borne as a recessive on the X chromosome. We are dealing with that 6.9 per cent, 1 in 14, of all men who have imperfect, but not absent, perception of red and green (Johnston, Cheeseman and Merrett 1957). For this purpose it is defined as inability to read the figures in the Ishihara test boo-k in good daylight as do normal individuals, and further as misinterpretation of the Ishihara figures in a way standard for the defect or defects. This test seems to identify a group of men each with a similar disability, though the disability cannot be identical because the group seems to include a few pure red blind, a few pure green blind, and a much greater number of men with lesser defects in red and green vision (Rushton, 1975). A more discriminating test would separate these groups and enable one to describe separately the disability in each. It should be noted that without formal testing, very few, or no, red-green blind men come to know they have the defect.