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K. S. LASHLEY 



ence the animals could distinguish. The size of the smaller 

 circle was increased to 40 millimetres, the 50 millimetre circle 

 being retained as the positive stimulus. Rat No. 4 learned to 

 distinguish between these circles, but after 200 trials grew in- 

 attentive. Punishment was introduced to strengthen the motive 

 and was continued throughout the third 100 trials (table X). 

 The punishment was not associated with the stimulus and 

 served only to confuse the rat so its use was abandoned. After 

 200 more trials the association was restored, but further attempts 

 to reduce the difference between the stimuli produced an un- 

 certain reaction. The smallest difference which the rats can 

 distinguish, under the conditions of the experiment, is about 

 two-fifths of the area of the larger circle. 



TABLE X 

 Discrimination Between Circles of Different Size. Rat No. 4 



EXPERIMENT 10 

 In the preceding experiments it appeared that the principal 

 fault in the method of experimentation lay in the fact that 

 the rats did not attend readily to the stimuli. Those which 

 learned to react to differences in size or form had first learned 

 to attend to brightness differences. From this it seemed prob- 



