100 COLOUR-BLINDNESS. [BOOK in. 



or less confound red and green ; but when a number of such 

 colour-blind persons are tested in making matches either between 

 skeins of wool or otherwise, it is found that they do not all 

 make the same matches; they do not agree as to the par- 

 ticular red and green which they regard as identical, and they 

 disagree in various other matches. But they all agree in this 

 that when they are tested by the method of mixing colours it is 

 found that all the colour sensations which they experience, 

 including white, may be reproduced by mixtures of two colours 

 only, whereas as we have seen ( 761) normal vision requires 

 three. For instance all the colours which they see may be 

 reproduced by varying mixtures of yellow and blue. The vision 

 of these colour-blind people is therefore dichromic not trichromic. 

 All their colour sensations are compounded of two not three 

 (or two, not three pairs of) primary sensations. 



On further examination it is found that these ordinary colour- 

 blind persons may be more or less successfully divided into two 

 classes. The members of one class have the following characters. 

 The spectrum seems to them shortened at the red end ; that is 

 to say they fail to receive any visual sensation from the rays 

 of extremely long wave-length which still give to the norjnal eye a 

 distinct sensation of red. The blue-green of the spectrum seems 

 to them less deeply coloured than the rest of the spectrum either 

 on the red or on the blue side ; this part gives rise in them to a 

 sensation like that caused by feeble white light; they have a 

 difficulty in recognizing any hue in it and they often speak of it 

 as grey, while in the remainder of the spectrum both to the blue 

 and to the red side they have distinct sensations of colour. We 

 may call this region of the spectrum the 'neutral band'; and it 

 is one of the characters of this class that they see such a neutral 

 band in the blue-green. They confound, as we have said, reds 

 and greens, but when asked to make an exact match between 

 a red and a green they choose a bright red and a dark green ; 

 they are more or less uncertain about all colours containing red 

 or green, and when asked to match a purple they generally select 

 a blue or a violet. 



To the members of the other class, the spectrum is not 

 shortened ; they receive sensations as far to the red side as does 

 the normal eye. They also see a ' neutral ' band, but this is placed 

 in the green, that is to say, nearer the red end than is the case 

 with the first class. When asked to make an exact match 

 between red and green they choose a dark red and a bright 

 green ; when asked to match a purple they generally select a 

 green or a grey. 



Persons whose vision belongs to one or other of these classes 

 are sometimes spoken of as ' totally ' colour-blind ; for there are 

 grades of difference between such a kind of vision and normal 

 vision, and some eyes may be called 'partially' colour-blind. 



