SIZE AND FORM PERCEPTION 101 



truthfully state that an animal was discriminating between a 

 circle and a triangle if it regularly chose the former even though 

 the triangle were seven times as large as the circle, but it is 

 quite improbable that the basis of discrimination in such a 

 case would be anything other than size. Since such points 

 were omitted from the written account, one suspects that the 

 technique of the experimenters was decidedly imperfect. Even, 

 had they taken the precaution of equalizing the sizes, they 

 would then have faced a problem still more difficult. Their 

 task would then have been to show that the discrimination was 

 not due to unequal stimulation of different parts of the retina. 

 Furthermore, no information is offered as to the method of 

 cutting these forms. It is a very difficult matter to cut green 

 peas after any regular manner. How can the observers be 

 certain that their subjects discriminated on the basis of form 

 rather than irregularities in the surfaces? They make no men- 

 tion of "check tests" to eliminate this possibility. It is because 

 they leave vital points like these unmentioned and apparently 

 unnoticed that one is led to class the experiment as a very 

 superficial piece of work. 



In the report by Breed on the reactions of chicks to form 

 stimuli n the conditions of the experiment have been accurately 

 presented. His chicks were given an opportunity to select a 

 circle when appearing along with a square. Both stimuli were 

 presented in a dark room by means of the illumination of screens 

 consisting of two plates of flashed opal glass, over which were 

 set mats of tin or cardboard containing the desired openings. 

 Three chicks were used in this study, one of which, No. 76, 

 "learned to discriminate two optical stimuli on the basis of 

 difference of form." 



Because his studies of the reaction of other chicks to similar 

 stimuli yielded negative results, Breed attributes the positive 

 reactions of No. 76 to a fortunate choice of subject. Unfor- 

 tunately, however, he seems to have made no control tests to 

 determine whether or not the distribution of light on the chick's 

 retina was influential. An inversion of a square would cause 

 no change in the distribution of light ; such a change might 

 have been produced by turning the square through 45 °. A 

 control test of this sort, however, is more easily made when a 



11 Op. cit., pp. 290-293. 



