178 



COLOUR VISION 



unable to discover any difference produced by dark adaptation in 

 colour matches with small central fields. A greenish yellow spot on a 

 red background appeared to match the background both for the light- 

 adapted and dark-adapted eye so long as it was fixated exactly. On 



05 O 0> CTJ O CT> Cl 



ocntocDCico-. — _._. 



oi oi CI o en 



05 Ol -t* CO 

 ■fw O) 



CO 



CD 



4^ 



(S 



o 



< 



05 



■ft 

 ■ft 



O) 



o 



CO 



Fig. 52. 



Scotopic values of the homogeneous lights. 



Scotopic 



values of the photopic equivalent red (642 ^/u) — blue (460-8 mm) mixture. Abscissae, 

 wave-lengths of the prismatic spectrum of gaslight ; ordinates, arbitrary scale, 

 (v. Kries and Nagel.) 



slight deviation (1° to 2°) of the dark-adapted eye the spot appeared 

 much brighter than the background. The scotanopic foveal area 

 subtends a visual angle of about 1-5° (Nagel), but its exact dimensions 

 are difficult to measure. 



