TWO SOURCES OF LIGHT 79 



On the writer's theory the following explanation of 

 these deviations should be given. The muscles moving 

 the head of the animal to the side of the weaker illumina- 

 tion, having a higher tension than their antagonists, bring 

 about a deflection of the animal toward the side of the 

 weaker light. As soon as its two photosensitive areas in 

 the head the animal has no eyes which are not parallel, 

 but inclined to each other are deflected from the perpen- 

 dicular upon the line connecting the two lights, the photo- 

 sensitive areas of the animal will no longer be struck by 

 the lights at the same angle, but on the side of the weaker 

 light the area will be struck at an angle nearer to 90 than 

 the photosensitive area exposed to the stronger light. 

 In this way the change in angle will compensate the dif- 

 ference in intensity of the two lights until the orientation 

 of the animal is such that the compensation is complete 

 and both photosensitive areas receive the same illumina- 

 tion. The animal will then continue to move in this 

 direction. 



Patten has computed the angle of the photosensitive 

 surfaces for these animals from the angle of their orienta- 

 tion under varying inequalities of illumination. 



This angle has been computed for the blowfly larva, using the "angu- 

 lar deflections " already ascertained. The magnitude of the angle may 

 bear no direct relation to the actual angle at which the sensitive areas 

 are located in the body of the animal, because of the many factors which 

 may modify the direction of the rays before they fall on the sensitive 

 surfaces. The significant test of the hypothesis would be the constancy 

 of the angle when computed from experimental data obtained under 

 varying conditions. 



The method of constructing such an angle is shown in Fig. 30, in 

 which the opposing lights are assumed to be of a two-to-one ratio of 

 intensity. The line AB is drawn perpendicular to the direction of the 

 rays of light. On the line AB, construct angle BO C equal to the actual 

 average angular deflection of the larvae at a two-to-one ratio of lights. 



