322 



K. S. LASHLEY 



the circles or to the difference in the brightness of the passages. 

 A few control tests (table VIII) showed conclusively that the 

 discrimination had been made upon the basis of the difference in 

 the brightness of the passage only. The rats had been choosing 

 the larger circle and when the relative brightness of the two 

 was changed they chose always the brighter passage, regard- 

 less of the size of the forms. The slight difference in brightness 

 was evidently more easily associated with the motive than was 

 the difference in the size of the circles. 



TABLE VIII 

 Control on Experiment 7 



Nature of the Stimuli 



Illumination of larger circle reduced; 

 brightness of passages equalized. . . . 



Size of circles equal; illumination un- 

 equal ; animals chose the brighter. . . 



30 and 50 mm. circles; the 30 mm. 

 more brightly illuminated 



30 and 50 mm. circles; the brightness 

 of the passages equalized by re- 

 flected light 



30 and 50 mm. circles; the floor of the 

 passage to the 30 mm. illuminated 

 brightly 



Rat No. 4 



Trials 



50 

 10 



10 

 10 



Per 

 cent 

 Error 



44 

 



40 

 60 



Rat No. 5 



Trials 



30 

 10 

 20 



10 



10 



Per 



cent 

 Error 



60 

 10 

 95 



60 



80 



Rat No. 6 



Trials 



10 



10 

 10 

 10 



Per 

 cent 

 Error 



50 



50 

 50 

 60 



EXPERIMENT 8 



The preceding experiment was continued with the element of 

 brightness eliminated. A 32 c. p. lamp was suspended six 

 feet above the experiment box, so that the two passages were 

 brightly illuminated (table IX, illumination 1). Later, the 

 intensity of this light was increased until the difference in the 

 brightness of the passages was reduced below the human thres- 

 hold and, consequently, much below that found to exist for the 

 rat (table IX, illumination 4). The 30 and 50 millimetre circles 

 were retained as stimuli. 



At first the animals failed to discriminate between the circles, 

 but after 100 trials No. 4 showed evidences of discrimination, 

 and in 200 trials learned to choose the larger circle accurately. 



