112 



BENZER ET AL. 



5deld per ionization is considerably below unity; a recent determi- 

 nation for T2 gave a yield of 1/20 (Watson, 1950). 



(2) Inactivation by Indirect Effect. This is due to toxic agents 

 produced by the radiation in the surrounding medium and can be 

 obtained when phage is suspended during irradiation in a medium 

 free of protective substances. In this case inactivation by indirect 

 agents greatly exceeds direct inactivation. Indirect inactivation is 

 caused by at least two different agents or groups of agents: one, 

 short lived, is detectable only by its action during actual exposure 

 of the phage suspension to radiation; the other, relatively stable, 

 is detected by the persistence of its effect after irradiation (Watson, 

 1950). 



The survival curve due to the short lived indirect agent is not 

 straight but shows a downward concavity at low doses becoming 



TIME (SEC.) 



Fig. 1. Inactivation of phages T2 and Ti by UV irradiation. The log 

 of the fraction of surviving phage particles as a function of the time of 

 irradiation at a distance of 82 cm from a G.E. germicidal lamp. 



