170 H. M. JOHNSON 



indicated by the positive field, and were punished by an electric 

 shock for attempting to enter the food-compartment indicated by 

 the negative field. In an earlier communication 2 I described at 

 some length the optical instruments used, and the methods of 

 preparing the stimuli and of training the animals. 3 I followed 

 these methods rigidly in the present work except in three par- 

 ticulars. It became necessary, when the differences in width 

 between the members of the two systems of striae became small, 

 to make the adjustments by hand, employing the micrometer 

 screw for the purpose, instead of using the string and pulley 

 mechanism. Further, in this work it is unnecessary that each 

 animal be required to compare the test-fields at the same minimal 

 distance as that prescribed for another animal, in order to make 

 the results comparative. I therefore eliminated the stops in 

 front of the alleys A 1 and A 2 of the Yerkes box (shown in figure 6 

 of the last article cited) which I had used in the work on the 

 chicks. This reduced to 50 cm. the minimal distance at which 

 comparison could be made without a choice being registered. 

 The actual distance usually employed by the birds seemed to 

 be between 50 cm. and 60 cm. If the difference in width of 

 the members of the two systems was large as compared with the 

 least effective difference for the individual, the animals tended 

 to choose without comparing except possibly from the position 

 they happened to occupy in the home-compartment when the 

 exit-door was raised and the test-fields exposed. I retained the 

 plate glass partition formerly used in the work on the monkey, 

 in order to limit his movements. In this work he occasionally 

 thrust his head into one opening in this partition and withdrew 

 it without choosing that alley. Such behavior was relatively 

 infrequent, and in such cases I did not exclude the responses. 

 In practically all the presentations in which he compared the 

 two fields before choosing, he inspected them successively with 



2 Johnson, H. M. Visual pattern-discrimination in the vertebrates. I. Prob- 

 lems and methods. This journal, vol. 4, 1914, pp. 319-339. 



3 The original drawing for Figure 1 in that article was lost or destroyed after 

 it had been mailed to the printer. A second drawing had to be prepared hastily 

 and was used. It contains an error which I beg the reader, in the interest of clear- 

 ness, to correct. The acute angle I, II, in the figure should be lettered o, and 

 the obtuse an^le I, II, should be lettered p 1 instead of p. The system of right 

 lines bisecting the obtuse angle should be lettered III. On page 330, in the phrase, 

 " the lines III bisect the angle I, II (o)," the symbol o 1 should be substituted 

 for the symbol p. My responsibility for the error is limited to carelessness in 

 copy and proofreading. 



