APPENDIX 513 



different laboratories without regard for the geometry of the ir- 

 radiation setup and obviates the need for calibration of the lamp 

 in energy units. It automatically corrects for loss of radiation by 

 absorption in the suspending medium. Luria (1947) has de- 

 fined the physiologic unit r as that dose which will inactivate all 

 but 1/^ of the phage particles; i.e., when/? //?o = 1/^ or log^j&o//? = 

 1, the phage has received 1 r of radiation. 



The energy content of the r dose for different phages varies 

 with the sensitivity of the phage to ultraviolet radiation. In the 

 case of phage T2, one r dose corresponds to about 50 ergs/mm. 2, 

 and this phage can be used to calibrate the output of an ultra- 

 violet light in energy units provided the lamp is nearly mono- 

 chromatic at 2537 A, as is the G.E. germicidal lamp. The value 

 of r = \/e for the proportion of survivors was chosen because at 

 this per cent survival, the average number of lethal hits/phage 

 particle is one. This follows because the distribution of lethal 

 hits among the population of phage particles under irradiation is 

 a Poisson distribution. That is, the proportion of unhit phage 

 particles, P{0) = p/p^ = ^~". When n (average number of hits/ 

 phage particle) = 1, then p/p^ = \/e = 0.3679. The course of 

 inactivation is followed by making phage assays in the usual way. 

 Samples are withdrawn after various doses of radiation, diluted 

 in broth and assayed for plaque-forming particles by the agar 

 layer technique. [To avoid photoreactivation of the UV-inac- 

 tivated phage (see following section) the assay plates should be 

 kept in the dark.] The inactivation is an exponential function 

 of the dose of ultraviolet light down to a survival of 10~^-10~'''. 

 In the case of some of the phages the inactivation becomes 

 markedly slower from this time on, for reasons to be explained 

 later. 



[Photoreactivation 



Dulbecco (1950) has shown that phage which has been inac- 

 tivated by UV^ irradiation can be reactivated (i.e., made able to 

 produce plaques) if exposed to sufficiently intense visible light 

 (3,000-5,000 A). This reactivation can take place only when 



