BEHAVIOR OF VERTEBRATES 447 



problems are then outlined: (1) the stimulus threshold for stri- 

 ation, i. e., the width of the individual bands on one field to 

 insure discrimination between it and another sensibly uniform 

 field; (2) the threshold difference for size and conversely for 

 nurnber of individual bands; (3) the difference threshold for 

 direction of bands; (4) the difference threshold for contrast 

 which is really the threshold for brightness. 



To produce the pattern two glasses were ruled with fine opaque 

 lines so that the width of the lines and the clear spaces were 

 equal. When these plates were rotated over each other there 

 was shown a series of dark and bright bands of equal width. 

 It is Cobb's apparatus slightly changed. With this was 'used 

 a modification of the Yerkes — Watson double photometer box. 

 The whole apparatus is excellent in conception and control, 

 but for the exact details the reader must consult the original 

 papers. 



In attacking the first problem mentioned above, Johnson 

 used a series of black and white horizontal bands of equal width 

 with respect to each other but whose absolute width could be 

 varied from one which was invisible to one which was plainly 

 visible without changing any other factor. The task set was to 

 distinguish this pattern field from a plain field having the same 

 area, form, range of wave lengths and luminous intensity. The 

 subjects used were dogs, monkeys and chicks. The details of 

 the experiments are full of interest. No positive results were 

 obtained from the dog as his behavior showed no sensitiveness 

 to differences of detail in visual objects. The experimenter, 

 however, reserves his conclusions on this subject. The visual 

 acuity of the monkey was very like that of the human subjects 

 used for comparison. The acuity of the chick was about one- 

 fourth that of the monkey. The author raised further questions 

 as to the method of experimentation the most significant of 

 which, it seemed to the reviewer, was that relating to the optimal 

 distance of the test field from the animal. The distance used 

 in the work reported was, uniformly, 60 cm. 



In some experimentation not nearly so well controlled as to 

 the different factors involved, Szymanski (35) reports some 

 work on the learning process in white rats. The apparatus 

 consisted of three connected parts, but, as parts one and two 

 were so very simple and gave such inconclusive results they 



