QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 265 



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Fig. 23 Geometrical construction so show that when the sensitive surfaces 

 are inclined to each other, a change in the axial position of the animal may equalize 

 the light energy operating on them, although the intensity in the opposite sides 

 of the field is unequal. The arrows indicate the direction, the numbers, the 

 intensity of the lights ; for construction and proof, see text. 



We may assume, therefore, that the power to balance unequal 

 bilateral stimulation, by a change in the position of the axis of 

 the body depends on the incHnation to each other of the photo- 

 sensitive surfaces. The mechanism is simple and effective, and 

 the explanation given fits the facts so far as they are known. 



If this assumption be true, it should be possible to compute the 

 angle of sensitive surfaces for an animal when we know its angle of 

 orientation under varying inequalities of illumination. This 

 angle has been computed for the blowfly larva, using the 'angular 



