400 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



We also find that a certain deuteranope requires 33 units of the 

 standard 670-8 to match the spectral 670-8, and 64 units of the 

 same standard to match the spectral 552. 



If deuteranopic vision be merely a reduction form, we ought to 

 be able to calculate from these data the stimulus values of the 

 intervening lights. Take as an example 591, which is matched 

 by 202 units of 670'8 plus 67 units of 552. We must change from 

 the arbitrary units of the normal system by dividing by 88*5 

 and 71 respectively, and then express in the deuteranope's intensity 

 system by multiplying by 33 and 64. The stimulus value should 

 be therefore : 202 x 33 ^- 88-5 + 67 x 64 -> 71 = 135 (approx.) units. 



This, then, is the stimulus value of 591 /j-fi expressed in terms 

 of the intensity values of 670-8 and 552 p/m. on the hypothesis 

 that a match good for a trichromatic eye is valid for a dichromatic. 

 The observed value was 137. In this way the following table was 

 obtained : ( 15 ) 



Stimulus Values for Deuteranope ami Protanope. 



A further, but less satisfactory test is afforded by the con- 

 struction of a colour diagram or " triangle " combining observa- 

 tions made upon the two classes of colour-blinds. As this would 

 involve the discussion of some rather technical details, and involves 

 certain assumptions of questionable validity, it will not be further 

 considered in this essay. 1 



1 See the memoir by v. Kries cite 1 in the last footnote, also Nagel's Handb., 

 vol. Hi., p. 162, and especially for the theory of a dichromatic "Fehlpunkt," 

 Helmholtz, op. cit., pp. 363, &c. I am deeply indebted to Professor v. Kries for 

 information respecting his work on dichromatic systems. 



