Chap, m.] SIGHT. 909 



normal eye ; the least coloured part of the spectrum, the 'neu- 

 tral band,' occupies about the same position as the green seen 

 by the normal eye, for here the red sensation and blue sensa- 

 tion 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 sensa- 

 tions, would lead to a general deficiency of vision ; for the lack 

 of one-third of visual sensations would be equivalent to a dim- 

 inution of the total illumination of external objects to the extent 

 of one-third, and this, unless we suppose that the normal eye 

 lives in a superfluity of light must, especially in feeble light, 

 lead to dim vision ; moreover a vision which has to trust to 

 two-fold differences must be less sure than one based on three- 

 fold differences, But this does not necessarily follow ; the two 

 remaining sensations might become more highly developed, 

 might so to speak expand in the absence of the third. Or we 

 may suppose, as indeed has been supposed, that in these cases 

 neither the red sensation nor the green sensation is absent, but 

 that the two sensations coincide. We may imagine that the 

 visual substance, or whatever it be, changes in which give rise to 

 the red sensation, is affected by light of different wave-lengths 

 in the same way as is the visual substance changes in which 

 give rise to the green sensation. This might be illustrated by 

 making the two curves for red and green in Fig. 150 coincide. 

 If the red curve were moved bluewards so as to coincide with 

 the green curve, the figure would illustrate a red-blind case ; 

 if the green curve were moved redwards to coincide with the 

 red curve, the figure would illustrate a green-blind case. And 

 as a matter of fact the general vision of colour-blind people 

 seems to be as good as that of normal eyes; moreover, within 

 the range of the colours which they can see, colour-blind people 

 are if anything more acute than most people ; though they 

 regard as more or less alike two colours which seem to the 

 normal eye wholly unlike, they can more easily detect minute 

 differences such as those of shade or tone, within each of the 

 two colours. 



§ 567. The phenomena, however, of these two classes of 

 colour-blind eyes can also be interpreted on Hering's theory. 

 In both of them the red-green substance may be supposed to be 

 missing, and their dichromic vision to be made up exclusively 

 of changes in the yellow-blue and white-black substances. Since 

 they are thus supposed to have neither red nor green sensations, 

 they must necessarily confound red and green ; and the smaller 

 differences, which, as we have seen, divide into two classes all 

 those which confound red with green may be explained as follows. 



