CHAPTER VIII 



TWO SOUKCES OF LIGHT OF DIFFERENT 

 INTENSITY 



THE writer observed that if heliotropic animals are 

 exposed to two equidistant lights of equal intensity they I 

 move in a line perpendicular to the line connecting the' 

 two lights. 287 ' 294 This has been confirmed by numerous 

 observers, e.g., Bohn on Littorina, by Parker and his 

 pupils, especially by Bradley M. Patten on the larvae of the 

 blowfly, by Loeb and Northrop on the motions of the larvae 

 of Balanus, and by others. The question arises : In which 

 line will an animal move when the intensity of the two 

 lights differs ? . When the animal is positively heliotropic 

 it should cease to move in a line at right angles to the 

 line connecting the two lights but should move in a line 

 which deviates toward the stronger of the two lights ; if 

 the animal is negatively heliotropic it should deviate 

 toward the weaker of the two lights. When the two eyes 

 are illuminated by two lights of different intensity, the 

 illumination in both eyes can become approximately equal 

 only when the eye struck by the weaker light is exposed 

 at a larger angle than the eye struck by the stronger 

 light. Under such conditions, the animal should be com- 

 pelled to move in a straight line which, however, is no 

 longer at right angles to the line connecting the two lights, 

 but which deviates to an extent determined by the differ- 

 ence in the intensity of the two lights. The case was 



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