INDUCTION OF PHASE SHIFT 



543 



Illumination 



Fig. 1 {Continued) 



the Starts of the photoperiods. By the third day, the mid-reactive 

 phases of each of the four populations was either 6 or 12 hr removed 

 from the other three. This starting condition is represented diagram- 

 matically by the rectangles at the left of each of the graphs in Figs. 

 1 and 2. Thus at midnight (2400 hr, left end of the abscissas) popula- 

 tion A was in the early nonreactive or scotophilic phase, population B 

 was in the middle to late scotophilic phase, population C was in the 

 early reactive phase, and population D was in the middle to late 

 reactive phase. 



At 0945, each of the four populations was removed from its com- 

 partment, and aliquots were transferred to 10-cm petri dishes that 

 served as exposure vessels. A germicidal lamp (15-w Westinghouse 

 Sterilamp G15T8) was employed as source, the irradiance at 254 m/A 



