SIZE AND FORM PERCEPTION 113 



The results of these tests for general ideas are briefly as fol- 

 lows: a chick which has been trained to choose a 6 cm. circle 

 and reject a 4 cm. circle will choose the latter when presented 

 with a 3 cm. circle. Likewise, it will choose a 9 cm. circle when 

 presented with a 6 cm. circle. In the one case, what has form- 

 erly been the sign for a negative reaction is accepted as the 

 sign for a positive reaction. In the other case, what has pre- 

 viously been the positive sign is rejected, when presented with 

 a larger stimulus, as a "shock sign." 



VII. SUMMARY 



Under the conditions of the present method, the chick's 

 threshold of difference in size perception lies between one-fourth 

 and one-sixth when the diameter of the standard circle is 6 cm. 



Earlier experimenters on the chick's perception of forms have 

 failed to eliminate all possible conditions for discrimination other 

 than the factor of form. The chick can discriminate between 

 circles and triangles and circles and squares which are equal in 

 area, but, with the conditions as described in this paper, none 

 of the subjects with which the present experiment has been 

 conducted were able to discriminate between visual stimuli on 

 the basis of form alone. Reactions to optical stimuli which have 

 been interpreted by observers as indicating form discrimination 

 are probably made on the basis of unequal stimulation of differ- 

 ent parts of the retina. If local inequality of excitations on the 

 retina be the basis of these reactions, then the apparent dis- 

 crimination of form by the chick is, in reality, a keen percep- 

 tion of size differences. 



In sharp contrasts with the reactions to form stimuli are the 

 responses to sizes. A chick can acquire a perfect circle-triangle 

 reaction, but control tests show that it has no general idea of 

 circularity in contrast with triangularity. On the other hand, 

 a "large-small" trained chick reacts positively to the larger of 

 two stimuli even though this particular stimulus had been the 

 "shock" stimulus in previous experiments. 



The order of importance of factors in the chick's vision is 

 size, brightness and general illumination, and form. 



