TWO SOUECES OF LIGHT 



77 



fleeted by M and M'. In order to make the ratio of inten- 

 sities of light from M and M' different, the observation 

 stage was put at un- 

 equal distance from M Y 

 and M'. The larvae were 

 made to record their 

 trails while moving 

 under the influence of 

 two lights and the devi- 

 ation of this trail from 

 the perpendicular upon 

 the line connecting the 

 two sources of light M 

 and M' was measured 

 with a goniometer (Fig. 

 29). The result of the 

 measurements of 2,500 

 trails, showing the pro- 

 gressive increase in an- 

 gular deviation of the 

 larvae (from the per- 

 pendicular upon the 

 line connecting the two 

 lights) with increasing 

 differences between the 

 lights, are given in 

 Table I. Since the devi- 

 ation or angular deflec- 

 tion was toward the 

 weaker of the two lights (the animal being negatively 

 heliotropic) the deviation is marked negative. 



Fia. 29. Diagram to show the method of 

 measuring trails. The lines xy and x'y' are 

 drawn through the trails at the points reached 

 marked by the arrows when the side lights were 

 turned on. The angle of deflection from this line 

 is measured by a protractor, P. The small figures 

 near the arrows indicate the number of wig-wag 

 movements made when the side lights were 

 turned on; 1st and 2nd refer to the sequence in 

 which the trails were run. (After Patten.) 



