RESEARCHES BASED UPON THE THEORY 



239 



of the total areas of these curves 1 . Equation (4) shows that the wave- 

 length of the light corresponding to the point of intersection is independent 

 of the amount or kind of the deficiency in colour sensation of the observer. 

 " The curves given in Fig. 68 depend on : 



" (1) The accuracy of Abney's sensation curves. 



" (2) The correctness of Abney's theory that in the case of the 

 ordinary types of total or partial red or green colour blindness the 



Fig. 68. Luminosity curves of persons having normal and reduced red and green sensa- 

 tions. Abscissae, wave-lengths of the prismatic spectrum of the arc light ; ordinates, 

 arbitrary scale. (Watson.) 



ordinates of one of the sensation curves are all reduced in the same 

 proportion, and 



" (3) The additive property which has been assumed and which 

 involves the corollary that the areas of the luminosity curves obtained 

 by normal and colour-deficient persons are the same. Thus, if it can 

 be shown that the observed luminosity curves of persons who are colour- 

 deficient agree with the calculated curves it is strong evidence in favour 

 of the correctness of the above three assumptions." 



1 The wave-length at which .nil the curves will intersect depends on the distribution 

 of light in the spectrum employed in the experiments, i.e. on the source of light. 



