176 



COLOUR VISION 



number of matches between a mixture of two spectral lights and mono- 

 chromatic light of an intermediate wave-length, and observed the effect 

 of diminishing the intensity of the light, v. Kries and Nagel 1 took 

 greater precautions as to the condition of adaptation and found that 

 a scotopic colourless homogeneous blue-green possesses a six or seven 

 times higher scotopic value than the photopic equivalent red-blue 

 mixture. The greatest difference is shown between homogeneous reds 

 and yellow-greens. The latter, from 544juyz violetwards, give with red 



Fig. 50 Photopic luminosity curve of a protanope (flicker method). 



Periphery luminosity curve of the same protanope. 

 Photopic luminosity curve of a trichromat (nicker method). 

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



exactly equivalent photopic values, whereas the achromatic scotopic 

 value of the yellow-green is more than 100 times that of the photopic 

 equivalent red (642 /z/x). These remarks apply to the deuteranope 

 and are illustrated in Fig. 52. Similar though less marked differences 

 occur in protanopes. 



The differences to which reference has already been made (Part I, 

 Section IV, Chap, i) between the photopic and scotopic periphery 

 values of the trichromat are accentuated in the dichromat. Colours 



. /. Psydiol. u. Physiol. d. Sinnesorg. xn. 1, 1896. 



