1 66 The Animal Mind 



experiments made practically no attempt to guard against 

 the possibility that the birds were reacting to differences in 

 the brightness of the lights. Another method used by 

 Katz and Revesz (395) was that of scattering grain on 

 grounds of different colors and noting how often the grain 

 was picked from the several grounds. The positive result 

 of this research was that while fowls with light-adapted 

 eyes pecked equally often at grains on yellow, green, red, 

 and violet grounds, those with dark-adapted eyes never 

 pecked at grain on the red ground. This indicates a pro- 

 cess of darkness adaptation like that in the human eye, 

 which sees red as very dark in faint light. In a later inves- 

 tigation the same workers tried scattering red, blue, and 

 green grains of various saturations, mixed with grains 

 stained four different shades of gray. All the grains were 

 stuck fast to the ground except those of a particular color. 

 The fowls showed an ability to discriminate which was 

 about equal to that of the normal human being. It can- 

 not be said, however, that these experiments satisfactorily 

 eliminated the brightness error, since so few shades of 

 gray were used. 



The effect of different colored rays on the pupillar re- 

 flex of birds was studied by Hess (303). For day birds, 

 he found that the maximal effect was produced by the 

 yellow rays ; for owls, by the yellow-green. That is, the day 

 birds showed the brightness distribution characteristic of the 

 light-adapted human eye with color vision ; the night birds 

 the distribution of total color blindness, or darkness adap- 

 tation. By his method of observing under what illumina- 

 tion the animals could find food, Hess obtained results lead- 

 ing him to conclude that day birds have a spectrum short- 

 ened in the violet end, a fact which he ascribes to the effect 

 of the oil globules in the retina ; and that the spectrum for 



