14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



2753 A. This ray proved lethal to the algae in three plates at an 

 exposure of 120 seconds, but no lethal regions for this ray appeared 

 in the plates exposed for 165 seconds. Consequently, 180 seconds 

 seems to mark the threshold of lethal action for 2753 A. thus giving 

 a radiotoxic spectral sensitivity value of .578. 



2804 A. An exposure of 60 seconds at an intensity of 3,570 ergs/sec. 

 cm^ produced lethal action, whereas the same exposure at 3,500 

 ergs/sec. cm' did not. Consequently, the first-named exposure seems 

 to mark the threshold, giving a radiotoxic spectral sensitivity value 

 of .467, which on the smooth curve becomes .505. 



2894 A. Lethal regions for this ray first began to appear at an 

 exposure of 120 seconds, but as they did not appear on the plates at 

 an exposure of 165 seconds, the threshold seems to be in the vicinity 

 of 180 seconds, which gives a radiotoxic spectral sensitivity value 

 of .370, or .350 on the smooth curve. 



2925 A. This ray, which is of very weak lethal value, did not kill 

 the algae at an exposure of 1,200 seconds, but did at an exposure of 

 1,920 seconds, which would give it a radiotoxic spectral sensitivity 

 value of .097. It seems more likely that the threshold would be mid- 

 way between 1,200 and 1,920 seconds, or at 1,560 seconds, which 

 would give a radiotoxic spectral sensitivity value of .120 or .200 on 

 the smooth curve. 



2967 A. The lethal threshold for this ray is at an exposure of 240 

 seconds, giving a radiotoxic spectral sensitivity value of .097. 



3022 A. A lethal region for this ray is faintly visible at 960 seconds' 

 exposure, thus giving a radiotoxic spectral sensitivity value of .010. 



3130 A. This ray did not produce a lethal effect even in the experi- 

 ment with 3-fold intensity. 



The radiotoxic spectral sensitivity values of the curve previously 

 determined by Meier (1934) when reduced to the same scale show 

 reasonable agreement with the new values for the radio-toxic spec- 

 tral sensitivity smooth curve as may be seen in figure i. 



RADIOTOXIC SPECTRAL SENSITIVITY AND RADIOTOXIC VIRULENCE 



As described in a previous paper, the lethal response of the algae 

 to the ultraviolet rays may be considered from two points of view : 

 as to the radiotoxic spectral sensitivity and the radiotoxic virulence. 

 The term " radiotoxic spectral sensitivity " relates to the certainty 

 of the lethal action, while the term " radiotoxic virulence " may be 

 used to describe the speed of the attack. 



The determination of the radiotoxic spectral sensitivity, that is, 

 the relative radiotoxicity of rays of different wave lengths when 



