908 COLOUK-BLIKDNESS. [BOOK in. 



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 4 partially ' 

 colour-blind. Moreover, even among these ' totally ' colour- 

 blind persons individual differences occur in each class ; indeed 

 not a few cases are met with which do not seem to fit into either 

 class, since they unite in themselves some of the characters of 

 each class. But even if we make allowance for these excep- 

 tions, the existence of the two classes with their respective 

 features seems to offer a strong support to the Young-Helmholtz 

 theory. In both classes vision is dichromic not trichromic, 

 that is to say according to that theory in both classes one of 

 the three primary sensations is missing. Since the character- 

 istic mistake which they both make is to confound red and 

 green, we may infer that the missing primary sensation is not 

 blue but either red or green. If we further suppose that in 

 the first class red is missing, in the second green, all the 

 features of the two. classes seem intelligible. 



On this view all the visual sensations which the first class 

 experience are made up of green and blue ; and their vision 

 might be represented by Fig. 150 with the upper curve (1) 

 omitted. Owing to the absence of the red sensation, the ex- 

 treme red rays hardly affect them at all. Since all their visual 

 sensations are made up of various mixtures of the primary green 

 and primary blue sensations, and since the sensation which they 

 call white light (whatever it may be when compared subjec- 

 tively with that of the normal eye) is the sensation produced 

 when rays of all the wave-lengths of the visible spectrum are 

 falling on the retina at the same time, that is to say when both 

 of the two primary sensations are being equally excited at the 

 same time, it follows that any particular wave-lengths which 

 equally excite both the two sensations should also produce a 

 sensation which to them is identical with that of white light. 

 Now the blue-green rays do excite equally both the green and 

 the blue sensation (cf. Fig. 150) , and it is just at this part of 

 the spectrum that these persons see the ' neutral band ' spoken 

 of above. Further, the matches which eyes of this class make 

 are such as we might imagine would be made if the sensation 

 of red were absent, and the two remaining sensations when 

 mixed together made white. Hence members of this class are 

 spoken of as being " red-blind." 



In eyes of the second class, since red is present though green 

 is wanting, the spectrum extends redwards as far as in the 



