330 R. DULBECCO [voL. 59 



Inactivation of the phages was accompHshed with a low-pressure mercury 

 discharge lamp (General Electric "germicidal" lamp, 15 watts), giving most of 

 the UV energy in the line 2,537 A. The output of the lamp was kept constant 

 by alimenting it through a "sola" stabilizer and by using it only after it had 

 been burning for at least 20 minutes. 



The stocks to be irradiated were diluted in phosphate buffer plus MgS04 and 

 exposed to the lamp at a 20-inch distance either in an open petri dish with con- 

 tinuous shaking (3 ml of phage in a 10-cm petri dish) or in a quartz cell 2 mm 

 thick with parallel faces. Relative measurements of the incident UV doses were 

 made in some experiments by timing the exposure; in other experiments rela- 



Wai-er 

 Bai-h 



^=^ 



Ligh-h 

 Filier 



Condenser 



[^ J H-5 Lamp 



Figure 1. Diagram of the apparatus employed for illumination in liquid. 



tive and absolute measurements were conducted with a calibrated Westinghouse 

 SM-200 meter with tantalum photocell WL-775. A dose of UV will be expressed 

 as seconds of exposure to the germicidal lamp. The reactivating light was used 

 in two different ways: 



Illumination on the plate. The plates, prepared by the agar layer method 

 (Gratia, 1936; Hershey et at., 1943), were exposed right side up to the light of 

 two parallel fluorescent discharge lamps, 40 watts each, at a distance of 12 

 inches at room temperature. 



Illumination in liquid. The apparatus used is illustrated in figure 1. A mercury 

 discharge lamp, medium pressure (General Electric H-5 lamp, 250 watts) was 

 used as the light source. The light was condensed through a spherical pyrex flask 



229 



