COLOUR-BLINDNESS. 1093 



these well-defined cases, others are met with which seem to show weakness 

 rather than absence of certain colour sensations. They behave in the 

 same way as in the complete form, but only confuse colours when they 

 possess a low degree of intensity or saturation. Such cases, as might be 

 expected, differ from the more complete cases in the limits of the 

 spectrum and in the form of their luminosity curves. 



Again, besides these intermediate cases, others occur in which there 

 does not seem to be any confusion of colours, but which are in certain 

 ways anomalous. These cases are often described as individual variations. 

 They were first observed by Eayleigh, 1 who found that if a number of 

 individuals matched a mixture of spectral red and green with spectral 

 yellow, some required a much larger proportion of green than the others, 

 while some, on the other hand, required a much larger proportion of red. 

 The yellows matched by such individuals appear very different to the nor- 

 mal eye. In addition to these marked variations, slight differences occur, 

 and these may often be found between the two eyes of the same individual. 



The subject has been fully investigated by Donders 2 and Bering. 3 

 Bonders examined the proportions in which light corresponding to the 

 lithium and thallium lines was mixed to match sodium light. In sixty 

 cases, fifty-six gave results agreeing fairly, the variations being within 

 moderate limits. In the other four cases the proportions were very 

 different, the proportion of green to red being very much larger, 48 -50 as 

 compared with 2 3 '7 7. Donders, however, unlike Eayleigh, found that 

 these cases showed a weakened colour-sense of the same kind as in the 

 common form of colour-blindness. Donders then examined several 

 cases of weak colour-sense, and found that, as regards the match in 

 question, five belonged to the second category, requiring a larger propor- 

 tion of green ; three other cases of weak colour-sense, on the other hand, 

 belonged to the first category ; while one case, belonging to the second 

 category as regards Eayleigh 's match, appeared to have normal colour- 

 vision. In the complete form of red-green blindness, it was found that 

 Eayleigh's match was no longer possible. 



Donders supposed that the results depended on a difference in the 

 relative intensities of lithium and thallium light, but found that this 

 was much less than could account for the condition. 



Maxwell 4 found that considerable variations occurred in the colour 

 equations made by different individuals, and referred them to corre- 

 sponding differences in macular pigmentation. This explanation has 

 been applied and developed by Hering, not only for individual varia- 

 tions, but also to explain the two forms of red-green blindness. His 

 investigations will be considered later in connection with his theory. 



Blue-yellow blindness. Cases in which there is confusion between 

 yellow and blue are very rare. The two chief cases have been recorded 

 by v. Vintschgau 5 and Hering, 6 and by Donders. 7 The first case 

 was very completely investigated. It was found that both yellow 

 and blue could be matched with grey and with each other, while a 

 match could not be made between red and green. The spectrum was 

 shortened at each end. There was a broad neutral grey band in the 

 yellow, extending from 598 \ to 568 X, and the short wave-end also pre- 



1 Nature, London, 1882, vol. xxv. p. 64. 2 Arch. f. PhysioL, Leipzig, 1884, S. 518. 

 3 Lotos, Prag, 1885, Bd. vi. S. 142. 4 Phil. Trans., London, I860. vol. cl. p. 76. 



5 Arch.f. d. ges. PhysioL, Bonn, 1891, Bd. xlviii. S. 431 ; and 1894, Bd. Ivii. S. 191. 



6 Ibid., 1894, Bd. Ivii. S. 308. 7 Brit. Med. Journ., London, 1880, vol. ii. p. 768. 





