QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 263 



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Fig. 21 A diagram showing that changes in the axial position of an organism 

 with parallel sensory areas, or eyes with tangents at the optical axes parallel, 

 do not change the relative amount of light intercepted by the sensory areas. In 

 this figure, the lights, indicated by arrows, are assumed to be equal in intensity 

 and opposite in direction. 



on a plane paratangential to their optical axes at the focal dis- 

 tance of the dioptric' apparatus. If these paratangents are 

 parallel, rotation of the axes of the system will not change the 

 relative amount of light intercepted by these planes. If, how- 

 ever, we assume that the sensitive surfaces are inclined at an 

 angle to each other, the case is different. Figure 22 is a diagram 

 constructed to show the conditions set up by rotation when the 



