68 ROBERT M. YERKES AND JOHN B. WATSON 



around the drum. The two ends of the cord are then forced 

 to run over fixed pulleys and are joined as described on page 

 56 and made to run freely through a hole in the metal bar sup- 

 plied with a clamp-screw, 87 in fig. 7. It must be remembered 

 that block 46 is free to slide on its track from right to left. 

 Suppose that block 46 is pushed to the left, and that the cord 

 around the drum is locked at 87 ; fig. 1 2 , C show^s what will happen : 

 first, note the position of the episcotisters in B, fig. 12, and 

 compare this with C, fig. 12, where block 46 has been made to 

 travel one-half its distance from right to left. It will be seen 

 that the drum 74 has been forced into rotation by the unwind- 

 ing of 89. At the same time, cord 80 has been made to wind up. 

 Note that in C, fig. 12, motor M and the two episcotisters have 

 moved one-fourth of a turn clockwise (in w^hich position the 

 episcotisters no longer interrupt the beams). ' In fig. 12, D, 

 block 46 has reached its extreme left position. Each cord is 

 now^ half wound up. The episcotisters are now reversed (as 

 are the right-left relation of the two colors). In fig. 12, B, (cor- 

 responding to position show^n in fig. 7) the green is on the right 

 and is interrupted by episcotister K, whereas the red beam is in- 

 terrupted by P. In fig. 12, D, the green beam is on the left and 

 is still interrupted by K; whereas the red is on the right and 

 is still interrupted by P. Referring again to fig, 12, B, suppose 

 that the cords 80 and 89 are not fixed and are allowed 

 to move freely through the clamp 87, fig. 7; as block 46 is made 

 to travel as before to its extreme left position at fig. 12, D, 

 the cords 80 and 89 will not wind and unwind as they did in 

 the other case, but will slip easily over their pulleys. The table 

 74 will not rotate, and consequently the episcotisters will inter- 

 change with respect to the two beams. In this case at fig. 12, 

 B, P will engage the green and K, the red. 



As might be inferred from the above description and from that 

 on p. 56, three possible ways of using the mechanism are open: 



(a) The sectors may be turned until they take the position 

 fig. 12, C, the cord being allowed to run freely through the 

 clamp 87, fig. 7. The full intensity of the beams is available. 



(b) The same sector can be made to interrupt a given beam 

 regardless of the latter's left-right position (in order to effect 

 this turn the sectors to the position B, fig. 12, and lock the cord 

 at 87, fig. 7). This adjustment must be open to us when we are 



