398 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



light undergoes selective absorption, and the homogeneous match 

 is nearer the red end. Thus the typical difference may be oblite- 

 rated and a simple determination of the neutral point would not 

 enable us to distinguish the two forms. 



A study of the neutral point, however, immediately brings to 

 our notice that characteristic which has attracted the attention of 

 practical men. Spectral light between 490 and 500 yu/z induces 

 normally the sensation green ; for the dichromatic it has the 

 same effect as unanalysed daylight or a mixture of red and blue. 

 Now this mixture contains much red and little blue, so that a 

 normal person asked to name the simple colour and the mixed 

 colour which look alike to the dichromatic would probably call 

 them green and red ; so that we can say that both protanopes 

 and deuteranopes confound red with green. Even here we observe 

 a class difference. In matching a given bluish-green, the prota- 

 nope, being relative insensitive to long waves, requires much red 

 in the red-blue mixture ; the deuteranope takes about the same 

 amount of blue but much less red. Accordingly, a protanope will 

 match a light bluish red (physically speaking) with a green that 

 appears to us much darker, e.g. a scarlet with olive green ; a 

 deuteranope matches a far bluer red with a green which we should 

 take to be about equally bright. Hence, although both groups 

 confuse certain reds with certain greens, the matches of one class 

 are not usually valid for the other. 



These then being the chief characteristics of the two common 

 forms of colour-blindness, we must endeavour to ascertain what 

 relations their visual systems bear to those of normal persons. 

 The mere fact that the systems are dichromatic would tell us com- 

 paratively little ; we should indeed conclude that, owing to the 

 reduction of different stimulus forms, sensations of colour are less 

 numerous for dichromatics than for ourselves, but they might 

 be quite different, and what we seek is a relation between 

 stimuli. 



As early as 1837, Seebeck expressed the opinion that two lights 

 or mixtures of lights which appeared equal to the normal eye, 

 never appeared unequal when examined by a partial colour-blind. 

 If this be true, it is of importance, because it would show that 

 these people lack something possessed by a normal person, but 

 have nothing, no stimulus reaction, which is not shared by the 

 normal eye ; in other words, colour-blindness is purely an error of 



