410 WILLIAM L, DOLLEY, JR. 



blackened eye, tending to cause movement toward the non- 

 functional eye. However, the lack of correspondence between 

 the size of the circles made and the intensity of non-directive 

 light shows that the organism does not respond to differences 

 in intensity as would be demanded by this theory. This con- 

 clusion is further supported by the fact that a gradual change 

 in the intensity of light does not affect the angle of deflection. 

 Consequently orientation can not be due to a balanced effect 

 of the action of the covering on one eye and the action of light 

 on the other. 



F. EFFECT OF ILLUMINATING ONLY ONE EYE 



1. Effect of illuminating the entire surface of one eye 



The experiments described previously in which one eye was 

 thrown out of function by being blackened were supplemented 

 by others in which one eye was prevented from functioning 

 simply by not being illuminated. This was accomplished by 

 methods somewhat similar to those used by Holmes and McGraw 

 ('13). These investigators held insects between the fingers over 

 a ''thin horizontal disc rotating on a pivot, like the turntable 

 of the microscopist," with the head pointing either toward or 

 away from the center. ''An electric light was so placed that 

 the rays of light fell upon one side of the body. These insects 

 attempted to turn toward the light, and by the action of their 

 legs, caused the disc to rotate in the opposite direction. When 

 the animals became quiet they could, generally, be caused to 

 resume their activity by pulling them slightly backward." 



Holmes and McGraw draw very decided conclusions from the 

 results of these experiments. They say (p. 373): "There can 

 be little doubt that light exercised a continuous stimulating 

 influence upon their [the butterflies] activity. It is not possible, 

 we believe, to construe phototaxis entirely in terms of differential 

 sensibility. Responses to the shock of transition, whether in 

 the direction of an increase or a decrease of stimulus, may play 

 a part in the orientation of many forms, but the continuous stimu- 

 lating influence of light appears to be, in several cases, at least, 

 the factor of major importance." 



