182 COLOUR VISION 



CHAPTER III 



ANOMALOUS TRICHROMATIC VISION 



Slight variations in the colour matches made by people with 

 apparently normal colour vision are commonly met with, and varia- 

 tions occur in the same individual under as nearly as possible identical 

 conditions at different times of examination. Setting aside physical 

 causes, such as differences in macular pigmentation, pigmentation of 

 the lens, etc., ample reasons for slight variations will be found in 

 successive induction, fatigue, the psychological state, and so on. 

 There yet remain, however, cases which cannot be explained on any 

 such grounds. The majority on exhaustive examination show inter- 

 mediate grades linking them with the normal on the one hand and the 

 dichromats (protanopes and deuteranopes) on the other. Konig called 

 this form of colour vision anomalous trichromatic vision. As Green- 

 wood saysi, " It is not improbable that these abnormal trichromatics 

 are extreme variants oi a frequency system representing the whole 

 range of visual types. The matter can only be settled when the 

 quantitative mixing ratios for a definite match have been determined 

 on a large number of persons taken at random ; we may then find 

 that the results are in accordance with some well-known frequency 

 distribution, the normal equation representing the modal value." 



We owe the discovery of probably the two largest classes of these 

 anomalous trichromats to Lord Rayleigh^. They approximate to the 

 protanopes and deuteranopes. Lord Rayleigh found that if homo- 

 geneous yellow (Z) line, 589 ju^u,) is matched with a mixture of homo- 

 geneous red {Li line, 670-8 /x/z) and homogeneous green {Th line, 535 /xju.) 

 some persons require much more red, others much more green than 

 the normal. The latter are the more numerous class and may be 

 called partial deuteranopes {deuteranomal, Nagel). Of them, Bonders^ 

 found four cases amongst 60 people examined ; Konig and Dieterici^ 

 3 amongst 70. Many cases have been thoroughly investigated by 

 Hering^, Lotze^, v. Kries'^, and Abney and Watson. Of the partial 



^ Physiology of the Special Senses, p. 137. 



2 j^Tature, xxv. 64, 1882 ; Brit. Assoc. Rep. 728, 1890. 



» Arch f. Anat. u. Physiol. 520, 1884. 



* Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. iv. 293, 1892. 



^ Lotos, VI. 1885. * Dissertation, Freiburg, 1898. 



' Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xix. 64, 1899. 



