SIZE AND FORM PERCEPTION 73 



mined. Part of the set I used, which consisted of 47 plates, 

 is included in table 1. Owing to the variations in sensitiveness 

 among different subjects, a set which would meet the require- 

 ments for one animal would not suffice for another, hence, in 

 those cases where the size differences among the plates vary by 

 one millimeter, a safe margin has been allowed by enumerating 

 a few more plates than would ordinarily be required, and those 

 plates for which I found no use have been omitted. 



4. Accessories 



Connected with both ends of the stimulus shifter, II, figure 

 1, page 67, are two ropes which pass over several pulleys and 

 terminate at H x and H 2 . These ropes hang 85 cm. back of 

 the table, T, thus leaving the experimenter an abundance of 

 free space at the front of the apparatus. They are sepaiated 

 from each other by no cm. The purpose of this attachment 

 is to enable the experimenter to change the position of the 

 shifter without leaving his place of observation. As is shown 

 in the initial figure, the shifter can be moved from its present 

 position to the right side by pulling on H 2 . H t would thus be 

 raised, and by pulling on it the adapter would be returned to 

 its present position. The stops v and v' are so adjusted to the 

 shifter that the openings in the brass plates will fall directly 

 in the line of illumination from the source box. These stops 

 are horizontally adjustable (not shown in the figure). The 

 center of the pulley which v carries is 92^ cm. above the floor. 

 The pulleys at I, J, K, and M are 230 cm. from the floor. The 

 distance from I to K and from J to M is 200 cm. 



Arranged upon the same frame is another system of ropes and 

 pulleys devised to control the positions of the source lamps. 

 The terminals of these ropes are indicated by G t and G 2 at the 

 experiment end, and by O l and 2 at the opposite end. The 

 drawing accurately represents the actual conditions at the 

 experiment end, but on account of the arrangement of the 

 walls of the dark room, O t , 2 , R, and P merely represent the 

 principle of the system. The ropes G r O x and G 2 -0 2 are cen- 

 trally attached to the source carriages of C and D. By lifting 

 slightly upon G l7 Oj begins to lower and draws back the lamp 

 carriage through C. By lightly pulling down upon G t , the carriage 

 in C is drawn forward. The system G 2 -0 2 works similarly. 



