J.— PSYCHOLOGY 215 



estimate. Both reports emphasise very strongly the fact that the anomalous 

 trichromate behaves as if colour blind when conditions are unfavourable, 

 such as when mist, fog or smoke are present, and this is all the more 

 disastrous because the individual is rarely aware that he suffers from any 

 colour defect. The heightened contrast and the quick fatigue, character- 

 istic of such anomaly, may well make the judgment of colour and the 

 discrimination of colour highly erroneous. The testing and discovering 

 of these cases of anomalous trichromatic vision is not easy, and necessitates 

 very careful procedure, and generally not the application of one test, but 

 of a battery of tests. 



The testing of colour blindness, indeed, in all its forms, presents a 

 problem of great practical importance, involving the adoption of a definite 

 technique. 



As is well known, the adult colour blind avails himself of all kinds of 

 secondary aids to enable him to discriminate colour. Identifying 

 marks on the surface such as differences in texture or slight defects in 

 material, or some minute details overlooked and undetected by the 

 normal observer are readily made use of, so that it is possible for a colour 

 blind who is aware of his abnormality to hide his defect in everyday life 

 if he wishes to, and he can very often do so with surprising skill. In 

 some cases, some sense, other than vision, may be utilised to help in 

 discrimination. One individual examined by the writer — a case of 

 shortened spectrum, in which one typical confusion is red with black — 

 experienced difficulty with inks of these two colours. In marking a 

 school register, he was constantly confusing the two colours, putting red 

 ink where black should be, and vice versa. The discovery that he could 

 distinguish them by smell solved his difficulties. 



This obscuring of colour defect came to light recently in the writer's 

 experience in a rather curious way. In copying pictures from originals — ■ 

 given the outline of the picture and a box of paints with the names of 

 the colours removed — the characteristic confusions generally appeared. 

 One individual, however, often obtained matches of colour which were 

 very similar to the original, and if only the final product were considered, 

 his colour vision would be thought to approximate very closely to 

 normal. To watch him at work was very illuminating, and one soon 

 learned that many of his excellent reproductions were based on deduction. 

 He knew foliage was green, for example ; he also knew that blue and 

 yellow mixed together produced green. His procedure was ' to take 

 what you think to be blue, and what you think to be yellow, and you 

 should get what is called green ! ' The green paint in the box, the exact 

 match of the green in the picture, was not used at all. 



An unusual case reported by Gildemeister and Dieter,^® is that of an 

 engineer who was classed as a typical protanomalous trichromate as the 

 result of an anomaloscope test, but who some time later passed the test. 

 As a trick was suspected, the anomaloscope was turned upside down, and 

 the knobs adjusted so that they had to be moved in the opposite direction 

 but still he was successful. It was surmised by the authors that he had 



" Arch. f. Ophth., 1922, 107, 26-29. 



