Report of the Committee on Colour- Vision. 329 



The Witness : It generally affects both eyes, and causes a 

 lowering, but seldom complete loss, of the functions of the central 

 part of the visual field. Except in very severe cases perception 

 of black and white remains, but there is, I believe, always a 

 disproportionate lowering of perception of colours over that area. 

 I believe the usual form it takes is blindness to the complementary 

 colours — red and green. 



For detecting the presence, and roughly estimating the size 

 and density, of the defective area (scotoma), it is enough to 

 use a small piece of coloured paper on the end of a stick or a 

 pen ; the coloured piece should vary in diameter from 5 mm. or 

 less, up to 25 mm. or more, according to the severity of the 

 case ; the more the sensibility to red, e.g., is lowered the larger 

 must the retinal image be, i.e., the greater the number of units 

 excited, in order that the sensation of red may be produced ; 

 also the greater the defect the brighter must the colour be. For 

 accurately mapping the scotoma of course the perimeter must be 

 used. As the loss of colour perception on the greater part of the 

 defective island, and often over the whole of it, is only partial, 

 the size of the scotoma and its exact outline, like the size and 

 exact outline of the normal field for any colour, vary according 

 to the size and quality of the colour used, and also to some 

 extent with practice and attention on the patient's part. 



I usually take red first, because any defect in that is most 

 easily apparent ; it is not so easy to get a pure green, and many 

 people are uncertain between blue and green, or clo not know 

 the names. In very slight cases, however, we sometimes use a 

 pale green in preference. The green I use is as pure a light 

 green as I can get. "Emerald green" conveys to me the idea 

 of a bluish-green, but perhaps erroneously. Light-green baize 

 would be the colour, I should think. 



Question — Could you describe the green you mean in wave- 

 length ? — I have no knowledge of colour expressed in terms of 

 wave-length. 



The detection of the scotoma depends in a certain degree on 

 the luminosity of the test-colour employed ; cast, par., the lower 

 the saturation of the coloured spot, and the smaller the diameter 

 of the coloured spot, the more easily is the defect perceived 

 (see answer to a previous question). 



I do not test with mixed colours : for instance, purple I found 

 unsatisfactory, except in slight cases of tobacco amblyopia, where 

 you must either take an extremely small spot of pure colour or a 

 larger spot of carefully mixed colour. Such patients will sometimes 

 say mauve is red. Something depends on the patient's training 

 and intelligence. I had a case of central scotoma from tobacco 

 smoking in a man who had been accustomed to deal in artist's 

 pigments ; he recognised every colour, pure and mixed, in spots of 

 various sizes, till I tried a dark sort of mauve, which appeared 

 to him blue or bluish in the centre of the field. He said that the 

 only commercial colour with which he had had any difficulty was 



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