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 



300 



200 



01 23456 

 O>&O>0>QO}OIU1O1OIO1 



OC7JfO03OlcO-i-*_i i|O 



7 8 9 10 11 12 13-5 15 16-5 18 19-5 21 22 23 24-7 



Ol Ol 

 K> - 



tn 01 



01 

 O 

 en 



(D 



en 



CD 



oo 



*. 



CD 



O 



A 

 O) 



to 



O) 



o 



Fig- 52. Scotopic values of the homogeneous lights Scotopic 



values of the photopic equivalent red (642 p./j.) 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. 



