194 CHARLES A. COBURN 



It seems probable that the birds were simply choosing the 

 more highly illuminated stimulus-chamber, which, in every case, 

 was also the one presenting the larger stimulus area. That they 

 did not continue to use this cue is proved by experiments in 

 which the large stimulus area, and irregularly the small one 

 also, were darkened by placing one thickness of milk glass over 

 the opal flashed glass. This enabled the experimenter in some 

 tests to present two stimulus areas differing in size and intensity 

 of illumination. Now the chamber illuminated by the larger 

 circular area was the more intense, and now the one illuminated 

 by the smaller area. Had the crows attempted to depend upon 

 the illumination of the chambers, or on the relative intensities 

 of the stimulus areas, instead of on their size, they certainly 

 would have been confused. As a matter of fact, the change 

 influenced markedly neither their behavior nor their percentage 

 of correct choices. 



The experiments on the perception of size were continued 

 for twenty-five days. The results (Table 2) show that the crows 



TABLE 2 



Perception of Size 



Correct choices 

 Date No. of tests Crow No. 1 Crow No. 2 



Conditions of Discrimination 

 5 centimeter — 2 centimeter circle 



