22 ROBERT M. YERKES AND JOHN B. WATSON 



This serves as a diffusing surface and constitutes the stimuli 

 area. It is held against the brass plate, s, by two screws, u, 

 fig- 3- (6) To the metal frame p is screwed from the front at w, 

 fig. 3, and three other points, a ring of hard rubber, n, i.8 cm. 

 thick, 2 cm. wide with a circular aperture lo cm. in diameter. 

 To this rubber ring is glued a ring of piano felt about | cm. 

 thick. These hard rubber and felt rings serve to fill in the space 

 between the metal front, z, of the light box, and the aluminum 

 plate, y, which carries the stimulus plates. (7) By means of 

 the bolts, at h and corresponding positions in the three other 

 corners, the stimulus adapter may be brought into the necessary 

 proximity to z to prevent light from passing from one stimulus 

 opening to the other, between z and y. Each of the four bolts 

 (h) at the corners of the stimulus adapter carries, between z 

 and y, a coiled spring, f, fig. 3, which serves to press y away 

 from z. By putting the proper amount of pressure on the four 

 springs the experimenter can so adjust the surfaces of the rings 

 of piano felt to the planed front of the steel plate z that the 

 light can not pass between the two, while, at the same time 

 the surfaces may be moved over one another freely whenever 

 it is necessary to move y. (8) At either end of the frame of the 

 stimulus adapter a stop, V, fig. i, is attached so that y shall 

 not run beyond the track T. 



The most important part of the " brightness " apparatus is 

 the set of accurately made brass stimulus plates which is briefly 

 described in table 3. 



The set, as used by us at present, consists of twenty-six plates. 

 Others may readily be added as they are needed. These plates 

 fall into three groups: (i) the light perception plates — three 

 plates with a 6 cm. circular opening and three with a 5 cm. 

 circular opening. This provides a plate for each of the three 

 windows of the stimulus adapter with a diameter of either 5 

 or 6 cm. (2) The size perception plates — this group consists 

 (including those of group i) of plates with circular openings 

 which, between 6 cm. and 5.5 cm., differ by i mm. in diameter, 

 and between 5.5 cm. and 3 cm. by 5 mm. ' (3) The form per- 

 ception plates — there are in this group four plates whose open- 



' In case an animal should prove al^le to discriminate slighter differences in 

 size than 1 mm. it would be necessary to cut additional plates, but this may 

 readily be done. 



