TWO SOURCES 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 pnt at un- 

 equal distance from M 

 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 f 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. 



Fig. 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.) 



