SIZE AND FORM PERCEPTION 71 



The construction of this mechanical device for opening E 

 appears in figure 3 of which T is the trip that was seen in the 

 other figures and t is a short piece of No. 30 black thread attached 

 to T at a. Passing down through the floor, F, of the electric 

 box, W, t runs over a pulley, P, and is fastened to the spring 

 lock, L, at b. B is a block through which L passes and against 

 which one end of the spring, C, presses. D is a stop attached 

 to L supporting the other end of C. This spring tends to force 

 L in the direction of X and to keep T in the position as illus- 

 trated. But when sufficient pressure is applied to T at any 

 point above R, L is forced back in spite of the pressure of C, 

 E is released at X, and the recoil of S raises E. When E is 

 drawn down, L, by reason of the slanting end surface at X, 

 is forced toward P until the notch of E has passed below the 

 lower side of L when the constant pressure of C toward X forces 

 L into the notch of E and the door is again locked. 



In several respects figure 3 is a very poor representation of 

 the tripping device. L is shown as constructed solidly in B, 

 when, in reality, it slides very freely through B. Moreover, 

 L is sadly disproportioned for it appears in the figure as wide 

 as the floor of the experiment box, but it is really a delicate 

 latch narrower than the width of T or E and is released by a 

 very light touch upon T. 



2. Source box 



To provide illumination for the two stimuli simultaneously 

 presented to the chick, the source box represented in figure 4, 

 — Ill of figure 1, — was used. A complete description of this 

 part of the mechanism appears in Behavior Monographs, loc. 

 cit., p. 17/jF. 



j. Stimulus shifter 



Figure 4 also presents a general view of the stimulus shifter 

 by means of which size, form, position, and, in conjunction with 

 the lamp carriages, brightness of the visual stimuli can be reg- 

 ulated. The details of the stimulus shifter or adapter appear 

 in the Yerkes and Watson report. 



A considerable number of standard brass stimulus plates, 

 used for varying the size and form of the visual stimulus, is 

 required if an animal's discriminative ability is to be deter- 



