188 CHARLES A. COBURN 



and Cole. The stimulus plates used in the experiments on size 

 and form, were the standard plates devised by Yerkes and 

 Watson s for their brightness vision apparatus and are described 

 in detail in their paper. 



The floor, walls, and top of the discrimination-chamber and 

 the two stimulus-chambers were painted a dark gray. This 

 rendered the two stimulus-chambers alike in every way except 

 with respect to the desired difference in optical stimuli, namely, 

 that of brightness, size, or form. Care was taken throughout 

 the work to see that this was the only means by which the crow 

 could choose the correct path. 



The exit doors were operated by a system of cords. A cur- 

 tain was suspended from the ceiling at the rear of the apparatus. 

 The experimenter, standing behind the curtain and looking 

 through a small peep-hole, could observe the behavior of the 

 crow while in the apparatus and open and close the exit-doors 

 without being seen by the crow. 



Late in the summer, two swinging gates of wire were sus- 

 pended between the discrimination-chamber and the two stimu- 

 lus-chambers. These gates also were operated by cords. At the 

 beginning of a test they were drawn up to the ceiling of the 

 discrimination-chamber. The purpose of these gates was to 

 prevent the crow from returning into the discrimination-chamber 

 after it had made a wrong choice. 



During the experiments on brightness discrimination, the 

 apparatus faced a north window. With the beginning of the 

 tests of size discrimination, it was shifted to face a larger south 

 window. In this position, it remained during the rest of the 

 season. 



The ability of the crow to detect a slight change in the situa- 

 tion, together with its wary and suspicious nature, made it 

 impossible to choose a method of procedure at the beginning 

 and to adhere to it rigidly throughout the period of work. The 

 method used at the beginning was evolved during the prelimi- 

 nary trials, when the first indications were received of what the 

 crow might reasonably be expected to do. Various changes 

 were made in this initial method until a reasonably satisfactory 

 one had been developed. 



* Yerkes, R. M., and' Watson, J. B. Methods of Studying Vision in Animals. 

 Behavior Monographs, 1911, vol. 1, no. 2, p. 23. 



