210 H. M. JOHNSON 



In this work I used the same experiment-box and test-objects 

 and the same general technique as are described in the former 

 report. 8 The mean brightness of each test -field was 6.2 candles 

 per square meter. The width of each stripe in either system was 

 3.902 mm. The animal had to choose with the eye 40 cm. or 

 more from the test-field, and he was fed in the food-box indicated 

 by the horizontal system. I gave him 20 trials each day. In 

 140 trials the animal chose correctly only 75 times, and showed 

 no tendency to improve-. There seemed reason to question 

 whether he was capable of discriminating between purely visual 

 objects, and of demonstrating the fact by acquiring a perfect 

 habit. Professor Mast, with whom I discussed the question, 

 urged me to settle the point. On June 24, 1915, I gave the 

 animal 20 trials in which only the positive test-object was pre- 

 sented. He was fed in the box at which it appeared. All 20 

 choices were correct, and there was no hesitation at any time. 

 On the following day I reintroduced the negative' field, but 

 stopped it down, so as to introduce a difference in area in addi- 

 tion to the difference in pattern. Hitherto, each field had been 

 limited by a circular diaphragm 6 cm. in diameter. Over the 

 diaphragm limiting the negative field I now slipped a smaller 

 diaphragm, the opening of which was concentric with that of 

 the former. The animal's daily records for the various differ- 

 ences in area appear in table 1. The results show that the 

 dog is able to discriminate when the difference of area presented 

 is between the ratios of 2.25 to 1 and 1.44 to 1. On July 24, 

 August 1, August 14 and August 31 I made control-tests, in 

 which the width of the striae on both fields was so reduced 

 that the fields appeared of uniform brightness to the human 

 eye. As will be seen in the table, these tests showed that the 

 dog's behavior was not affected by the presence or absence of 

 the stripes. After the animal had ceased to discriminate, when 

 the stimuli was a 6 cm. circle and a 5 cm. circle, I retrained 

 him for six days, beginning August 30, 1915, requiring him to 

 choose the 6 cm. circle and to reject the 3 cm. circle. Both 

 fields were of uniform brightness. His average percentage of 

 correct choices for the 120 trials was 85.8. I then reduced the 

 brightness of the 6 cm. circle by introducing an absorption- 



8 In the present work I placed a copper tray, holding a moistened felt pad, between 

 the exit-door of the home-box and the alleys enterin? the food-boxes. This tended 

 to reduce the variability of the resistance of the animal's feet to a minimum. 



