APPENDIX 511 



mutants cannot. Thus any wild type progeny that arise in 

 crosses between different r mutants can be found by platings on 

 K-12, as only they will form plaques. The total yield from the 

 cross can be obtained by platings on coli B. In this way recom- 

 bination frequencies as low as 10~^ could be measured, the 

 only limitation being the reverse mutation rate of the parental r 

 strains.] 



The exchange of genetic characters occurs not only between 

 mutants of the same phage type but between those of different 

 phage types. In fact, the exchanges were first observed by Del- 

 briick and Bailey (1946), who carried out mixed infections with 

 r"*" and r forms of T2, T4, and T6 in various combinations. For 

 instance, mixed infection of B with T2r+ and T6r yielded T2r 

 and T6r+ in addition to the 2 parental types. . . . 



Effects of Ultraviolet Irradiation of Bacteriophage 



A most convenient and generally accessible source of ultravio- 

 let light is the General Electric germicidal lamp, a low pressure, 

 mercury vapor lamp with about 80 per cent of its ultraviolet out- 

 put at a wavelength of 2537 A. This wavelength is very close to 

 that found most efficient for sterilization of bacterial suspensions 

 and inactivation of viruses (Gates, 1 934) . The lamp can be op- 

 erated in any standard fixture designed to take fluorescent lamps 

 of the same dimensions. A metallic reflector should be avoided, 

 for it will reflect ultraviolet light and complicate the geometry of 

 the irradiation set-up. The lamp must be shaded so that no di- 

 rect rays reach the eyes of anyone in the vicinity. The ultravio- 

 let output varies with the line potential, so that for reproducible 

 results a constant voltage transformer such as the Sola,* with out- 

 put of 0.5 amp. at 115 v., should be used with the lamp. For 

 most work this is not necessary. The ultraviolet output varies 

 slightly during the useful life of the lamp. This can be con- 

 trolled only by calibration of the lamp by photoelectric or bio- 

 logic methods. The ultraviolet dose varies directly with the 



* Made by Sola Electric Company, 4627 West Sixteenth St., Cicero, 111. 



