THE COMPARATIVE PSYCHOLOGY OF COLOUR VISION 1 1 1 



generally pecked somewhat farther towards the blue, but seldom in 

 the blue and violet. When the spectrum was reduced until it was almost 

 colourless for the dark-adapted human eye they pecked chiefly or solely 

 in a region corresponding to the yellow and orange-yellow, i.e., rather 

 red-wards of the human scotopic maximum luminosity. Chickens 

 48 hours after hatching behaved in the same way as older birds. Hawks 

 (Falco tinnunculus), fed with pieces of meat, showed the same shortening 

 of the violet end of the spectrum. All the birds first pecked in the region 

 which is brightest for man, and then sought regions of less luminosity. 

 If half the field were illuminated with light transmitted through a red 

 glass and the other half with blue light, light- or partially dark-adapted 

 birds pecked first in the red half, turning to the blue half only when 

 the red was cleared of grains. By suitable alterations of the strengths 

 of the lights, the blue half was left untouched, even though the red grains 

 appeared to the human eye very dark red and scarcely visible whilst 

 the blue were clearly seen. Blue grains, however, were as eagerly pecked 

 as red if the light were made sufficiently strong. Dark adaptation 

 increases the sensitiveness of birds' vision very markedly. The " peck- 

 ing-limit " is for them only slightly, if at all, higher than the extrafoveal 

 threshold visibility of the grains for a 1 2 hours' dark-adapted human 

 eye. Adaptation appears to be slower in hawks and buzzards than in 

 the human eye. 



Nocturnal birds were investigated in the same manner. If an owl, 

 which has perched for several hours in sunlight, is examined with a 

 moderately bright spectrum illuminating white pieces of meat, these 

 are snapped up first in the red, yellow, green, or blue-green. The 

 spectrum extends further violet-wards for nocturnal than for diurnal 

 birds, but is still much shortened at this end. When thoroughly dark- 

 adapted owls snap at meat in the blue and violet ; they snap first in that 

 part of the achromatic spectrum which is brightest for man, or slightly 

 farther towards the violet. Dark adaptation causes a rapid rise of 

 sensibility during the first half -hour and the maximum sensibility 

 appears to be much greater than for man. 



The pupil reactions of birds are somewhat complicated. The 

 maximum constriction for diurnal birds (fowl and pigeon) is in the region 

 of the orange and yellow, diminishing gradually towards both ends of 

 the spectrum, more rapidly towards the violet the less the dark adapta- 

 tion. For the dark-adapted eyes of nocturnal birds the maximum is 

 in the yellow-green and green, falling slowly towards the violet end. 



It appears therefore that diurnal birds see the spectrum shortened 



