CHAP, in.] SIGHT. 101 



Moreover, even among these ' totally ' colour-blind persons indi- 

 vidual 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 exceptions, 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 characteristic 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. 146 with the upper curve (1) 

 omitted. Owing to the absence of the red sensation, the extreme 

 red rays hardly affect them at all. Since all their visual sen- 

 sations 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 subjectively 

 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. 146); 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 normal 

 eye ; the least coloured part of the spectrum, the ' neutral band ', 

 occupies about the same position as the green seen by the normal 

 eye, for here the red sensation and blue sensation are excited to 

 about the same extent; and the matches made by eyes of this 

 class are such as might be expected in the absence of the green 

 sensation. Members of this class are accordingly spoken of as 

 " green-blind." 



It might appear at first sight that the lack of a primary 

 sensation, that is to say, the want of a third of all visual 



