DICHROMATIC VISION 



177 



which, when viewed by peripheral parts of the retina, accurately 

 match under light adaptation and with high intensities of light, may 

 possess very unequal achromatic scotopic values, and the inequality 

 is much greater with dichromatic than with trichromatic vision. 



Regional effects. As might be expected the dichromat, like the 

 trichromat, sees colours best with foveal stimulation. For certain 

 conditions, indeed, his powers of discrimination with central vision 



Fig. 51 Photopic luminosity curve of a deuteranope (flicker method). 



Periphery luminosity curve of the same deuteranope. 



Photopic luminosity curve of the same trichroraat as in Fig. 50 



(flicker method). 

 Periphery luminosity curve of the same trichromat. (Polimanti.) 



are markedly superior to those of the normal. Thus, in the protanope 

 a yellow-green light (545 ^ufi) possesses at least 100 times the achromatic 

 scotopic value of its photopic red (670 ^ or higher) match, whereas 

 in the normal the maximum ratio is 6:1. He is, therefore, in a 

 peculiarly advantageous condition for determining whether the fovea 

 is sensitive to changes of adaptation. Nagel 1 , a deuteranope, was 



1 v. Kries and Nagel, op. cit. xxm. 161, 1900. 



P. c. v. 



12 



