HELIOTROPISM 61 



"All the experiments show that the muscle tone is de- 

 pendent upon the intensity of the light and that the 

 postures assumed depend upon the relative difference in 

 the light stimulus to the eyes. In animals with one eye 

 completely covered the radii of the circles in which they 

 moved were shorter the more intense the illumination of 

 the normal eye. With one eye partially covered the 

 circles were larger than when completely covered, and in 

 the same way the circles were larger when one eye was 

 covered bv a film of collodion or of brown shellac, which 



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admits some light, than when subsequently covered by 

 opaque asphalt black. When one eye was partially cov- 

 ered by central application of the black paint the tilting 

 and circling to the opposite side were abolished or re- 

 versed by brilliant illumination of the partially blackened 

 eye. These results explain why a positively heliotropic 

 animal with one eye blackened approaches a light by 

 a series of alternating small and large circles, the former 

 being executed when the good eye is illuminated from the 

 source of light, the larger when it is in the shadow." 



We have thus far discussed chiefly positively helio- 

 tropic animals, i.e., animals which are compelled to move 

 toward the source of light. The difference between these 

 and negatively heliotropic animals is that the legs on the 

 illuminated side of a negatively heliotropic animal are 

 extended, while those on the opposite side are in flexed 

 position. This has been directly observed by Holmes, 

 who also made sure of the fact that negatively heliotropic 

 animals, when one eye is blackened, turn in circles with 

 the blackened eye toward the center of the circle 228 ; while 

 positively heliotropic animals turn in circles with the 

 unblackened eye toward the center of the circle. 



