ACTION OF ULTRAVIOLET LIGHT ON VIRUSES 1()5 



tion was made by Luria and Dulbecco (1949). For low doses, 

 where r <<C //, the value of y is 



assuming that t)nly two phage particles are absorbed per bac- 

 terium. The value of r can be obtained from the dose-survival 

 curve for single infection. At the 37% figure, r = 1, and, in 

 general, if n/no (the survival ratio) is measured, this is e~''. 

 With known values of r, infections which average two per 

 bacterium can be made, and the value of the multiplicity reac- 

 tivation can be found by comparison with the single-infection 

 curve. The value of y is determined from the excess survival for 

 double infection. The following figures are obtained. 



Phage r \,ij 



T^2, T^2r 



14 3 93 45 



The results, therefore, indicate that something like 50 essential 

 units are present in each virus. Although this may be treated as 

 no more than a figure to guide thought, it seems clear that the 

 multiple character of virus structure is required by these 

 experiments. 



The action spectrum for this process would be of the greatest 

 interest. 



Although there is no necessary relation between this effect 

 and the hypothetical structure deduced for T-1 phage from the 

 lengthening of the latent period, it is interesting that a somewhat 

 similar type of internal pattern is required for the two. 



Photoreactivation of Bacteriophage 



Further evidence that bacterial viruses are partly inactivated 

 by ultraviolet light, and that in this condition they still possess 

 some function, is found in the property of photoreactivation dis- 



