CHAPTER V 



Size Perception 



The foregoing discussion of method suggests the difficulties 

 that arise when one's experimental problem becomes limited 

 to a single detail of vision. It emphasizes the uncertainty of 

 earlier work where exacting controls were not employed. In 

 the light of my experience, it seems to be a safe guess that prior 

 studies, presumably of detail vision, were, in fact, studies of 

 reactions to visual complexes the constituents of which were 

 not known to the experimenters. The control, in my study, 

 leading to the discovery of the disturbing crack proves that the 

 results with chick 3, prior to the control, were untrustworthy. 

 In other studies where the possibilities for similar disturbances 

 have been many times greater, results should be accepted with 

 considerable caution. 



After the discovery of the light-admitting crack, my results 

 become more reliable. Later experiments with chick 3 indicate 

 that the early positive choices had been made on the basis of a 

 discrimination of size difference, but as the variable became 

 larger, and the discrimination proportionately harder, the chick 

 resorted to a different criterion for choosing. In table 2 one finds 

 evidence of the place where chick 3 ceased to discriminate be- 

 tween the two size stimuli. After the controls, the chick was 

 first tested with a variable o 23 + . The results are clearly nega- 

 tive since chance alone is sufficient to explain the number of 

 correct responses. With the variable reduced too 19 + , there 

 are only three correct choices in ten tests, but when the variable 

 was changed to o 15+ the results are clearly positive. The 

 standard was chosen nine times and the variable only once. 

 While the average results of the tests with the o 19+ are 

 slightly in favor of the larger stimulus, it seems that one should 

 not emphasize the fact for the larger circle was chosen, in a total 

 of 60 tests, only five more times than chance allows. 



Table 3 summarizes the results of my study of the chick's 

 perception of size stimuli. In contrast with earlier reports 



