J.— PSYCHOLOGY 209 



may also exist a form of monochromatism in which the visibility curve 

 is of the light-adapted or photopic type.^^ 



The second variety of colour blindness, which has little practical 

 significance, although it has considerable theoretical importance, is that 

 of blue-yellow blindness, in which the ability to see blue and yellow is 

 affected, but the ability to see red and green is unaffected. This second 

 type is of doubtful existence as a congenital condition, although one 

 or two apparently authentic cases have been recorded. Generally, it is 

 accompanied by some pathological change in the eye. One case, reported 

 by Richardson, saw blue as a dazzling white. 



Parsons ^* points out that ' it is simulated in cases of jaundice and 

 sclerosis of the crystalline lens, these being due to absorption by yellow 

 pigment.' Abney ^^ and Hess ^® have found that the same defect may 

 arise in cases where the pigmentation of the macula is unusually dense. 



This second form of colour blindness has been termed violet-blindness 

 by the adherents of the Young-Helmholtz theory, or as Maxwell preferred 

 to term it, blue-blindness. The followers of the Hering theory describe 

 the defect as blue-yellow blindness. The term, tritanopia, was suggested 

 by V. Kries. 



The third form of colour blindness, red-green blindness, is the most 

 important of all, because of the frequency with which it occurs, as has 

 been already observed, and because of the colours which are confused. 

 The defect is congenital and hereditary. It occurs on the male side of 

 a family, and is practically non-existent on the female side, only about 

 I in 500 being colour-blind. A female, however, whose father is colour- 

 blind, may transmit the defect to her sons. In other words, it may pass 

 over a generation, remaining latent in the female, but reappear in the 

 next generation on the male side.^' 



Two forms of this colour defect are recognised, depending on whether 

 the spectrum is of normal length or shortened. These are described as 

 green-blind and red-blind respectively by the adherents of the Helmholtz 

 theory : as red-green blind by the exponents of the Hering theory, the 

 difference in the two types being ascribed to differences in macular 

 pigmentation. The names deuteranopia and protanopia, suggested by 

 v. Kries, are used independently of any colour theory. The terms 

 photerythrous (visibility of the red) and scoterythrous (darkness of the 

 red) put forward by Rivers, have unfortunately never been adopted. 



It seems more or less agreed that red-green colour blindness is a 

 reduction system of normal colour vision, one cogent argument in support 

 of this contention being that normal colour matches are valid for any 

 type of dichromate. The colour blinds lack something which the normal 

 eye has, but have nothing which the normal eye does not possess. The 

 individual with normal colour vision sees a spectrum composed of six 



9 



^^ See Chapter 13 by Troland in A Handbook of General Experimental Psychology, 

 1934. Edited by Murchison. 



" Ibid., p. 192. 15 Proc. Roy. Soc, 1891, 49. 



i« Arch.f. Anat., 1908, 61. 



1' Colour blindness may also be acquired, but it is not necessary to discuss this 

 form of colour blindness here. 



