OTHER THEORIES 287 



the red substance is associated physiologically with the red stimulator 

 only, the receptor for medium waves in the green substance with the 

 green stimulator, and the receptor for short waves in the blue substance 

 with the blue stimulator. That is, each receptor transfers its whole 

 energy to the corresponding stimulator. 



In the early developmental stages other associations are possible. 

 In the totally colour-blind zone each receptor is associated with each 

 stimulator, so that each receptor divides its stimulating energy between 

 the three stimulators. Hence no matter what the stimulating light the 

 resultant sensation is white. In the red-green-blind zone the short 

 wave receptor is limited to the blue stimulator, whilst the two others 

 are associated indiscriminately with the other two stimulators. Hence 

 in this case the red and green stimulators always initiate the sensation 

 of yellow. 



It was suggested by Schaum that the receptors act as optical sensi- 

 tisers, and by Richarz that they may be regarded as optical resonators. 



Deuteranopia. This type of colour blindness shows the following 

 characteristics : 



I. It is a reduction-system from normal colour vision. 



II. It agrees thus far with the red-green-blindness of the middle 

 retinal zone that (1) affected people describe three sensations, white, 

 yellow and blue, and (2) their sensations are the same in quality and 

 nearly so in luminosity as the sensations of white, yellow, and blue in 

 regional red-green-blindness. 



III. It differs from regional red-green-blindness in that (1) it is 

 also present on direct fixation, and (2) it is not relative, since increase 

 of area, intensity, and saturation of the object never arouses the fully 

 developed colour-sense. 



Hence it is to be regarded as an arrested development, in which the 



second phase I 1 \ ] has not taken place. 



' 



The strongest arguments in favour of this view are the equivalence 

 of colour matches for the normal and deuteranope (v. p. 170) and the 

 almost complete identity of the normal and deuteranopic luminosity 

 curves. 



Protanopia is a reduction-system which differs from deuteranopia 

 and normal colour vision in the difference of the luminosity curves. 

 Lights of long wave-length have strikingly low stimulus values for the 



