196 



BIOLOGICAL EFFECTS OF RADIATION 



dows employed. Thus, in the case of cesium, thaUum, and uranium, 

 Fig. 32, (3), the sensitivity continues to rise from 3000 to 2400 A, as shown 

 by the dotted Unes where a quartz window replaces the customary thin 



2000 



3000 



WAVE LENGTH 

 4000 



5000 



6000A 



2500 



3400 



380O 



4200A 



2feOO 2700 2800 2900 3000A 2600 3000 



WAVE LENGTH 

 Fig. 32. — Relative sensitivity of special photocells: Sec. (1) alkali metals (Hughes and 

 DuBridge); Li, Na, K, Rb (Pohl and Pringsheim); Cs {Campbell and Ritchie). Sec. (2) and 

 Sec. (3) Westinghouse ultra-violet photoelectric cells. (Rentschler.) Full-line curves in 

 thin glass; dotted curves in quartz. 



glass window. From this group, a photocell can be selected having its 

 long-wave-length threshold anywhere from 2675 to 4200 A. By com- 

 bination with short-wave cut-off filters indicated in Table 10 and Fig. 9, 

 any required short-wave-length limit can be secured. Hence, by a 



