STATEMENT OF THE THEORY 217 



in which the stimulus values for any light of one of the components is 

 uniformly less than normal. Partial protanopia is then a diminution 

 of sensitiveness of the R component, partial deuteranopia of the G 

 component. Other modifications are theoretically possible and are 

 included in the general form that if in the normal system ^ = /i {x, y, z) 

 and so on, then in the anomalous trichromatic system (f)' =/i' {x, y, z) 

 and so on. This generalisation is, however, too vague to be of much 

 practical value, and there are other possibilities arising directly out of 



Bx 



V.515 



AX 



Fig. 57. Normal trichromatic colour diagram constructed from combined protanopia 

 and deuteranopic observations. A, null-point of protanopes; B, null-point of 

 deuteranopes. (v. Kries.) 



the Young-Helmholtz theory, such as a shift of one of the curves {vide 

 infra), which account for at least one group of the anomalous trichromats. 

 Monochromatic vision may be explained on the Young-Helmholtz 

 theory by the identity of all three components. In this case 



4> -fi (»' 2/> 2) = X =/2 i^> y,^) = ^ =h {^. y^ 2)- 



The facts do not support this view, and are much more satisfactorily 

 explained by the duplicity theory. At the same time there are some 



