J.— PSYCHOLOGY 211 



an investigation into colour blindness, found considerable difference 

 between the two eyes in many cases, and suggest that although there is 

 no conclusive evidence as to the frequency of unilateral colour blindness, 

 that ' possibly it amounts to three or four per cent, of the colour blind.' 

 They discovered one such case in a group of 23, and Miles and Craig 

 found another in a group of 390 mercantile salesmen.^^ This latter 

 individual was a dry goods salesman and aware of his condition, and if 

 required to match fabrics he closed the defective eye and made the 

 proper discrimination with his normal eye. If these cases of unilateral 

 colour blindness are as numerous as suggested, a field seems opened up 

 for further investigation which may lead to definite conclusions as regards 

 the fundamental sensations of the red-green colour blind. 



The results of investigations carried out over a number of years at 

 Edinburgh into the colour sensations of the red-green colour blind, seem 

 to indicate that their colour sensations are indeed blue and yellow.^^ 

 The extent to which these two colours replace all the other colours in 

 the spectrum is, however, not constant for every colour blind. With 

 some, yellow and blue may replace all the other colours, that is, red, 

 orange, yellow and green may be seen as different shades of yellow, blue 

 and violet as different shades of blue. Sometimes, instead of the green 

 being replaced by yellow, it may appear as grey, or the blue-green may 

 appear as grey, the extent of the neutral band varying with the gravity 

 of the defect. A second neutral band is also found extra-spectrally in 

 the complementary colour, in the purples. 



In one extreme case which came under observation, only two narrow 

 bands of yellow and blue existed, the other colours being seen as shades 

 of grey. Red appeared as black, orange as dark grey, yellow could be 

 discriminated, green and blue-green were seen as grey, blue could be 

 distinguished, but violet appeared as dark grey. In one test, out of 

 76 colours observed, 64 of them appeared as grey. 



All cases, however, are not so extreme as the one just described and the 

 general finding which seems to be gradually gaining ground, is that there 

 exist different degrees of colour blindness extending from extreme cases 

 in which blue and yellow are the only two colours visible to milder cases 

 in which the blindness to red and green is not total. In these milder 

 cases the individual can see red and green if they are bright enough or 

 vivid enough. It is these milder forms which constitute the great 

 practical problem, for in these cases the ability to distinguish between 

 red and green is likely to fail when the individuals are fatigued, or when 

 the illumination is poor, as in mist or fog. The extreme cases can be 

 easily detected, but the milder cases, which have been described as 

 ' dangerous colour-blinds,' require very careful examination if they are 

 to be detected. 



That different degrees of colour blindness exist has been emphasised 

 by more than one writer. Hayes ^^ ' wonders if we shall presently have 



^* ' Colour Blindness in Dry Goods Salesmen,' The Personnel J., 1931, 9, 437- 

 449. 



^^ Collins : Colour Blindness. 

 23 Psy. Bulletin, 1926, 23. 



