REACTIONS OF CHICKS TO OPTICAL STIMULI 283 



in the presence of the stimuli. The black cardboard shutters 

 M and M' were kept closed over the exits E until the animal 

 had selected one or the other of the two stimuli. Thus the 

 only light that reached a chick while in the act of discrimina- 

 tion was transmitted through the glass screens mounted in the 

 sliding frame F. In the size and form experiments these screens 

 consisted each of two plates of flashed opal glass, depolished 

 toward the lamps, glossy-surfaced toward the chick. The illum- 

 inated areas of various sizes and shapes were produced by black 

 mats of tin or cardboard that contained the appropriate open- 

 ings and were set in the frame over the glass on the side away 

 from the lamps. In the color work colored glass was inserted. 

 The frame was adjusted on brass rollers that were constructed 

 so as to work noiselessly. The lamps in the illumination box, 

 mounted as they were on moveable cast-iron bases, could be 

 shifted back and forth along the metric scales to provide such 

 variation in the relative brightness of the stimuli as the demands 

 of the experiment dictated. 



The dark-basket mentioned in the following account was a 

 willow basket lined with black cloth and covered with the same 

 material. It was used for the purpose of dark-adapting the 

 chicks. The animals were kept in this receptacle before and 

 between trials during all the form and size work, and during 

 all the color work except the preliminary tests. 



METHOD 



A chick that was being tested was introduced into the appa- 

 ratus by hand at A, whence it passed through the gateway B, 

 and walked toward one of the two lighted areas. If an error 

 was made, the animal received an electric shock, controlled at 

 K by the experimenter, whereupon it soon learned to pass to 

 the other side of the compartment and secure its release. If a 

 correct selection was made, the shutter M or M' was withdrawn, 

 exit E was thereby slightly illuminated with the dim light 

 reflected from the hover box, and the animal found its way 

 down the inclined plane G to heat, food, light, and water. There 

 was a possible source of error in the method at this point against 

 which the operator had to take precaution constantly. He made 

 it a rule not to withdraw the shutter until a chick was well onto 

 the wires in a position to be shocked. Occasionally a chick would 



