SIZE AND FORM PERCEPTION 81 



It was quite common for chicks that were being trained to 

 go beyond the door to the nest box and crowd up to the stim- 

 ulus at the end of the compartment. If the animal were allowed 

 to do this it spent considerable time and energy trying to get 

 through the illuminated stimulus plate. A thin plate of glass 

 was placed across each compartment at a point just beyond 

 the exit. The chicks could thus see through the stop but could 

 not go beyond it. They thus were stopped near the exit so 

 that they might learn more readily how to escape. As soon as 

 the}'' had acquired a mode of escape and had ceased trying to 

 reach the stimulus, the glass plates were removed. 



After a perfect habit had been acquired the amount of differ- 

 ence between the two stimuli was decreased. When the 

 o 28+ — o 7+ discrimination, for example, was perfect, the con- 

 dition was changed to o 28+ — 9+. The large circle was thus 

 the standard which remained constant. The smaller circle was 

 the variable, which, after each perfect reaction, was succes- 

 sively increased in size until the animal could no longer choose 

 correctly. This limit of correct choices was thus accepted as 

 the chick's threshold of difference. 



Table 1, page 74, provides for a series of stimulus plates 

 that will enable the experimenter to make the most satisfactory 

 changes in the variable stimulus. The changes in diameter can 

 well be as great as 5 mm. between o 7+ — and 012+ when used 

 with a standard o 28+. Above o 12+ the diameter of the 

 variable should be successively increased only by 1 mm. The 

 set of plates with which I started included a group between 

 o 19+ and o 28+ varying in diameter by 1 mm., but provided 

 only a o 16 between o 12+ and o 19+. A similar scale was 

 provided between o 50+ and o 63+ but there were no variables 

 between o 28+ and o 50+. After a little preliminary work, 

 however, I discovered that a different scale of variables was 

 necessary, and in table 1 I present the sizes which my experi- 

 ence leads me to regard as essential. Unless he wishes to make 

 a study of that particular problem, the experimenter, in using 

 variables differing in diameter by only 1 mm., must be alert 

 to see that the natural discrimination of his subjects is not 

 improved by training. 



During the experiments with the first group of chicks, I was 

 seeking to get a method as much as to train the birds. I can 



