VISION 387 



It is well recognized that there are various degrees of colour- 

 blindness, and that the colour-vision of persons considered 

 normal presents different grades of refinement. Nevertheless, 

 the abnormalities of colour-blind persons are so marked that 

 cases fall into definite classes. Those whose cones do not 

 function which means that their yellow spots are either 

 undeveloped or diseased see all things grey. They are totally 

 colour-blind. Excluding these, the colour-blind may be 

 grouped in one or other of two divisions (a) those who confuse 

 red and green, (b) those who confuse yellow and blue. One 

 person out of every thirty-five is red-green blind. The pro- 

 portion is even higher if males only are considered, showing 

 how very unfortunate is our choice of warning signals. A man 

 who is red-green blind cannot tell the port from the starboard 

 light. Blue-yellow blindness is, on the other hand, extremely 

 rare. According to Young's theory, colour-blindness is due 

 to the absence of one of the three sets of visual apparatus. 

 But cases do not altogether conform to this hypothesis. We 

 knew an amateur water-colourist, since deceased, who derived 

 intense pleasure from the beauties of Nature, and showed no 

 mean skill in reproducing them with his brush, notwithstanding 

 the fact that he was red-green blind. Each night his sister 

 arranged his paint-box for him, and only rarely did he use 

 vermilion to fill in a foreground of lush green grass. But this 

 mistake, when he made it, did not destroy his own satisfaction 

 in the picture. It was clear that red had a value for him, 

 although he confused it with green. It is impossible for a normal 

 person to see through the eye of one who is colour-blind, and 

 there is no other means of comparing his sensations with our 

 own. The mistakes which the colour-blind make in sorting 

 coloured objects and in naming mixtures of light selected from 

 various parts of the spectrum show the range of their de- 

 ficiency, but give us no information regarding the qualities 

 of the sensations which they retain. 



The test of colour-sensitiveness usually employed is the 

 grading of a large number of wools of different tint. The order 

 in which the colours should be arranged is not a matter of 

 opinion. They must be placed in the order in which they 

 occur in the spectrum i.e., arranged according to their wave- 

 lengths. In the cases of colour-blindness which are most fre- 



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