414 WILLIAM L. DOLLEY, JR. 



of light to strike the right eye from different directions. In one 

 experiment the beam struck the eye obhquely from the rear as 

 is represented by the arrow (a) in figure 21. Thus the posterior 

 half of the right eye was illuminated while the left eye was in 

 darkness. In another the rays of light struck the right eye 

 obliquely from in front, as is represented by arrow (b) in figure 

 21. 



Under the former conditions, when the posterior half of the 

 right eye was illuminated, eight animals were tested, each for 

 thirty minutes. As in the preceding experiment these insects 

 showed alternate periods of rest and activity. During the periods 

 of activity they all attempted to turn toward the illuminated 

 eye, as was shown by the direction in which they re\^olved the 

 disc* 



Fig. 21 Diagram to represent the direction from which beams of light were 

 allowed to strike the right eye (see text). 



Under the latter conditions, w^hen the anterior half of the right 

 eye was illuminated, the animals were also alternately quiet 

 and active. When active they varied in their behavior. Some 

 attempted to turn toward the illuminated eye, while others 

 attempted to turn in the opposite direction. They were tested 

 on several successive days, the tests on each day lasting for 

 thirty minutes. 



The first animal (A) tested attempted to turn continuously 

 toward the shaded eye on the first day. On the second day, 

 during the first part of the test, it attempted to turn in the same 

 direction, but during the last part of the test it attempted to 

 turn in the opposite direction, i.e., toward the illuminated eye. 



^ The disc could not be seen. Its direction of motion was i)erceived, however, 

 by means of a light paper arrow glued to the bottom of it. The hand of the 

 o))server was so held that the arrow struck the hand as the flisc revolved. 



