158 THE PHYSICS OF VIRUSES 



Careful studies have been made by Fluke (1953) on the action 

 spectrum of T-1 phage, dry at room temperature, at liquid- 

 nitrogen temperature, and wet at room temperature. In the dry 

 state, it is found that the inactivation at any one wavelength 

 follows the familiar relation 



In n/no — —al 



where 7i/iiq is the survival ratio, I is the total energy incident 

 on the specimen, and a is a constant depending on the amount 

 of energy absorbed and the quantum yield in the process. In the 

 wet state this relation sometimes holds, but in general it does not. 

 The type of curve fits a so-called "multiple-hit "process. The 

 general formula for an inactivation requiring ni inactivating 

 events for each of A^ necessary parts of a virus, with a probabil- 

 ity a per unit total dose applied, is 





= m— \ 



Li l-\ 



where h is an integer. (Timofeeff-Ressovsky and Zimmer, 1947; 

 Zirkle, Marchbank, and Kuck, 1952.) When only one necessary 

 part requires one inactivating event, we have the familiar sur- 

 vival curve n/riQ = e~°'\ For two inactivating events, the two- 

 hit curve is ri/no = e"""^(l + al), and so on. The multiplicity of 

 hit is not definite and depends on the virus and the wavelength 

 used. If an accurate survival-ratio curve is obtained, the num- 

 ber of hits can be ascertained and a measured. 



For the relatively simple case of dry irradiated T-1, Fluke 

 (1953) obtained the results shown in Fig. 6.8. The relative 

 action on infectivity is plotted on a logarithmic scale versus the 

 wavelength. This effectively means plotting a versus wavelength 

 for survival. The virus was irradiated both at room temperature 

 and liquid-nitrogen temperature. Slight but not very definitive 

 differences are observed. 



It is interesting that the action spectrum does not entirely 

 parallel the absorption spectrum. Most noteworthy is the fact 



