HELIOTKOPISM 57 



other. Bringing one eye into the bright beam of light 

 directed through the objective of the optical system of the 

 string galvanometer, while the other eye is illuminated 

 only by the subdued light of the optical room, promptly 

 produced the same changes in the position of the legs 

 and body which were observed when one eye was black- 

 ened, the more weakly illuminated eye acting like the 

 blackened eye in the former experiment. When the illu- 



FIG. 21. Position of robber fly when the lower halves of both eyes are blackened. Head 

 tilted up. (After Carrey.) 



mination on one side of such animals is stronger than on 

 the other the legs on the more strongly illuminated side of 

 the animal are bent, those on the opposite side are ex- 

 tended ; and the head has a tendency to bend toward the 

 light. When an impulse to move originates in the animal, 

 it can turn easily to the light but with difficulty in the 

 opposite direction. As soon as its head is turned to the 

 source of light and both eyes receive the same illumination 

 the difference in tension of the legs on the two sides of 

 the body disappears and now the animal moves or is 

 carried in a straight direction toward the light. By these 

 experiments the proof of the writer's muscle tension 

 theory of heliotropism is made complete. 177 



