STUDIES IN SPECIAL SENSE PHYSIOLOGY 399 



defect not excess. The validity of normal equations for the two 

 groups of partial colour-blindness has been recently tested by 

 v. Kries. His conclusions are clearly expressed in the following 

 passage : 



" One employs the frequently cited equations between a homo- 

 geneous yellow and a mixture of red and yellowish-green (670-8 and 

 550 /x/x). As all lights in this region are of equal stimulus value 

 for the colour-blind, whatever be the ratio of red : yellowish-green, 

 one can always give the homogeneous yellow an intensity such 

 that the match is good either for a protanope or a deuteranope, 

 but in general the matches of the one are not valid for the other. 

 As we should expect, a strongly red mixture is for the protanope 

 equivalent to a yellow of relatively feeble intensity ; a deuteranope 

 finds in a match arranged by the protanope, the mixture too 

 bright and the pure yellow too dark. The relation is reversed for 

 strongly green mixtures. With extraordinary accuracy, however, 

 we find that for the ratio of red : yellowish-green that has for the 

 trichromatic an equal colour tone with the homogeneous yellow, 

 both groups agree ; trichromatic equations are valid for both 

 protanopes and deuteranopes. Conversely, if we try to find an 

 equation valid for both groups, we arrive precisely at the one 

 valid for a normal person." l 



Additional evidence is afforded by the following considerations. 

 Suppose we prepare a table giving the mixing ratios for a normal 

 person in matching spectral colours between 670-8 /x/x and 550 ti/x. 

 The results are as follows : 



1 Nagel's Handb., vol. iii. p. 160. 



