512 BACTERIOPHAGES 



time of irradiation and approximately inversely with the dis- 

 tance of the sample from the tube, so that the dosage can be var- 

 ied within wide limits quite conveniently. The energy output 

 of a G.E,. germicidal lamp calibrated by Luria and Latarjet 

 (1947) was 16 ergs/mm. ^ sec. for the 2537 A band at a distance of 

 56 cm. from the lamp. Output varies from lamp to lamp but is 

 usually within 20 per cent of this figure. 



The sample to be irradiated should be diluted in a chemically 

 defined medium free from nucleic acids and other substances 

 which absorb strongly near 2537 A. The medium should be so 

 diluted that no mutual screening of phage particles or bacteria 

 can take place. Depth of the sample under irradiation should 

 not be more than 1 or 2 mm., and the sample container should 

 be mounted on a flexible support so that the sample can be con- 

 stantly agitated during irradiation. A Petri dish is a convenient 

 vessel, and irradiation can be started by removing the cover and 

 stopped by replacing it, since a glass Petri dish absorbs essentially 

 all ultraviolet light at 2537 A. 



For the coliphages T1-T7, inactivation by ultraviolet light at 

 2537 A is an exponential function of the dose of irradiation (Lu- 

 ria, 1947). This indicates that inactivation is probably a "one 

 hit" mechanism, 1 quantum presumably being effective if ab- 

 sorbed in a vulnerable site. 



Under constant experimental conditions the rate of inactiva- 

 tion of phage follows the equation 



\og/-^ = Kt 

 P 



This leads directly to a "physiologic unit" of irradiation in which 

 the dose of ultraviolet light is expressed in terms of the inactiva- 

 tion of phage rather than in terms of ergs or other energy units. 

 This is important in that it permits duplication of experiments in 



