J.— PSYCHOLOGY 213 



an excess of green. Von Kries, in describing an extensive series of 

 experiments, applied the name ' anomalous trichromates ' to these cases, 

 and this designation has gained universal currency. Guttman advocated 

 the terms red-weak and green-weak, and distinguished seven characteristics 

 which they manifest, the chief of which are high thresholds, heightened 

 colour contrast and quick fatigue to colour stimuli. 



It is customary to divide these anomalous trichromates into two groups 

 corresponding to the two groups of dichromates, deuteranomalous 

 trichromates or partial deuteranopes in which the sensitivity to green is 

 below normal, and protanomalous trichromates or partial protanopes in 

 which the sensitivity to red is below normal. The green anomalous 

 seem to be more numerous than the red anomalous, the ratio being 

 quoted as 5 to i. 



These cases, it may be, form the connecting-link between normal 

 colour vision on the one hand, and colour defect on the other, and if 

 a sufficient number of cases could be tested ranging from normality to 

 complete red-green deficiency, a continuous series might be obtained. 

 Below is a graph showing the results of testing 200 individuals, 100 women 

 and 100 men with the Rayleigh equation, with rotating discs.^^ 



The results are expressed as logarithms of the ratio of red to green. 

 For the 100 women tested, all the cases fall within the limits of the normal 

 curve. In this respect, the results agree with those of Houston -^ who 

 tested with spectral lights, and who found of 105 women students, no 

 scattered cases outside the normal curve. 



The curve, in the case of the 100 men tested, presents diflferent features. 

 It is difficult to know where the normal curve ends and abnormality 

 begins. The cases lying at the limits of the normal curve may be the 

 extreme variants, or may be just outside it. Out of 423 men students 

 tested, Houston found 15 outside the normal curve exclusive of colour 

 blinds. 



Edridge-Green ^" seems to apply the term anomalous trichromate to 

 the extreme variants within the normal curve. Among 100 women, he 

 found 14 who were anomalous, which means that he is describing cases 

 as anomalous which lie within the normal curve. Other investigators 

 apply the term only to the scattered cases outside the normal curve. It 

 may be legitimate to use the term either way, but the definition of 

 anomalous trichromasy will vary according to the usage of the term. 

 If the anomalous trichromates lie outside the normal curve, then the 

 difference between them and the normal may be qualitative, whereas if 

 the anomalous trichromates lie within the normal curve, the diff^erence 

 may be only quantitative. It may be, as I have suggested elsewhere,^^ 

 that as results accumulate, the normal cui-ve will spread out to reach these 

 isolated cases, and the gaps between will fill up. Kollner,^^ for example, 

 reports transition cases between anomalous trichromates and dichromates, 

 although none between anomalous trichromates and normal. 



^* Article by Collins : British J. Psychology, 1929, 14, 387-393. 



^* Proc. Roy. Soc, 1922. 



»» Ibid.. 86B, 164. 31 Loc. cit. 



32 Arch.f. Augenheilkunde, LXXVIII. 



