VISUAL PERCEPTION OF THE CHICK 67 



other than form. The results from the Inversion of the tri- 

 angle indicate that the basis of choice depends upon the unequal 

 stimulation of different parts of the retina. When the extended 

 base of the triangle is so placed as to stimulate the region of 

 the retina which was formerly stimulated by the apex of the 

 triangle, the chick becomes confused. Under the conditions of 

 the present experiment, therefore, I am forced to conclude that 

 the apparent reactions to forms are the result of keen percep- 

 tion of relative size differences. 



The image formed on the retina of the chick is truly changed 

 when the triangle is inverted by causing certain particular size 

 reversals. The vertex, which was formerly uppermost, becomes 

 interchanged with the base. The portion of the retina which 

 was formerly stimulated by an extended line of light relative 

 to the base of the triangle becomes stimulated by a mere point 

 of light which is related to the vertex of the triangle. Inversion 

 thus causes particular changes in the extension of the stimulated 

 portions of the retina. It would seem, therefore, that the in- 

 version of the triangle, though causing no change in form as 

 such, produces these relative size differences which were evidently 

 a source of confusion to the chick. 



This control of inverting the triangle is a severe test, but it 

 affords an excellent basis of comparison between the chick and 

 other birds. Coburn made the same test with his crows with- 

 out interrupting their correct choices. It therefore seems to be 

 a legitimate and highly desirable procedure in form studies. In 

 addition to circle-triangle discrimination, Coburn's crows dis- 

 criminated a circle from squares and hexagons. 



Coburn's crow study again furnishes from the birds the only 

 results directly comparable with my chick results. Porter tried 

 to study the perception of forms and designs in sparrows by 

 using six form boxes made of poplar boards. He secured nega- 

 tive results but his method was weak. His birds were free to 

 approach all of the boxes in one of which there was always food. 

 There was no strong incentive for the bird to seek the correct 

 box first because it was ultimately rewarded with food regardless 

 of its first approach. Even had the results been positive 

 nothing definite could have been asserted about the detail vision 

 of the birds on account of the lack of any system of control. 



