Chap, hi.] SIGHT. 907 



(Glasgow) college gown when it was lying on the college grass- 

 plot ; the colour-blind can tell a cherry among the leaves on a 

 tree much more by its form than by its colour ; and when such 

 persons are asked to ' make matches ' between coloured objects, 

 such as skeins of coloured wools, they will put together a red 

 skein and a green skein as being of the same colour. Most 

 colour-blind people more 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 particular 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 (§ 564) 

 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 sensa- 

 tions 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 normal 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 



