16 ROBERT M. YERKES AND JOHN B. WATSON 



tative methods — but it does not and can not, as ordinarily 

 need, enable us to determine with accuracy the limits of an 

 animal's ability to perceive difference in size. In other words, 

 it does not lend itself to the thorough investigation of size 

 perception. 



We wish to recommend a method whose chief defect, appar- 

 ently, is unnaturalness, and whose prominent advantages are 

 accurate controllability and describability. 



This, in brief, is the method. In a dark-room, and by means 

 of the apparatus described below in Section III, two visual 

 areas are simultaneously exposed to view. These areas differ 

 only in size, and with respect to this character they are under 

 the control of the experimenter. Differences of the compared 

 stimuli as to form, color, light, texture, position, odor, which 

 ■exist and in varying degrees influence the results obtained by 

 -common methods, are largely or wholly excluded by our stan- 

 dardized apparatus. 



2. Form perception 



The statements made concerning size perception apply also to 

 form perception. The procedure recommended, and for which 

 the necessary apparatus is described in Section III, p. 1 7, involves 

 the use of standard plates in which are cut openings in the 

 shape of circles, hexagons, squares, and triangles. 



3. Distance perception 



Few students of vision have considered the problem of dis- 

 tance perception in animals.' This is rather because of the 

 difficultness of the task than because the problem lacks interest 

 or importance. We have made some preliminary attempts to 

 devise a satisfactory method of dealing with the subject, but 

 we are not prepared at present to make recommendations. It 

 seems to us especially important that this visual factor be studied 

 in connection with color vision. 



1 Thorndike, E. L. The instinctive reactions of young chicks. PsychoL Review, 

 1899, vol. 6, p. 284 ff. 



Waugh, K. T. The role of vision in the mental life of the mouse. Jour. Comp. 

 Neurol, and Psychol., 1910, vol. 20, p. 572 ff. 



Yerkes, R. M. Space perception of tortoises. Jour. Comp. Neurol, and Psychol., 

 1904, vol. 14, p. 17 ff. 



