CHAPTER V 



HELIOTROPISM 



THE INFLUENCE OF ONE SOURCE OF LIGHT 

 1. GENERAL FACTS 



THE fact that certain animals go to the light had, of 

 course, been known for hundreds of years, but this was 

 explained in an anthropomorphic way. Thus Lubbock, 

 and Graber, 180 had taken it lor granted that certain 

 animals went to the light or away from it on account 

 of fondness for either light or darkness, and their experi- 

 ments were calculated to demonstrate this fondness. 

 Animals were distributed in a box, one-half of which 

 was covered with common window glass, the other with 

 an opaque body or with colored glass, and after a while 

 the number of animals in each half was counted. When 

 the majority of animals were found in the dark part the 

 animals were believed to have a preference for darkness; 

 when in the light part they were believed to be fond 

 of light. The same method was used to decide whether 

 animals preferred blue to red or vice versa. The writer 

 attacked the problem from the physical viewpoint, assum- 

 ing that the animals are "fonoT" neither of light nor of 

 " darkness, " but that they are oriented by the light in a 

 similar way as plants are ; being compelled to bend or as 

 in the case djf motile algae move automatically either to a 

 source of light-, or away from it. 285 ' 287 



In the case. of unequal illumination of the two eyes the 

 tension of the. symmetrical muscles in an animal becomes 



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