NO. 2 EFFECT OF ULTRAVIOLET ON ALGAE MEIER I5 



applied with equal intensity and duration, has been measured as 

 described in the preceding section. The determinations for each of 

 the ultraviolet rays when plotted against wave lengths give a curve of 

 radiotoxic spectral sensitivity. ( See table 7 and fig. i ) , 



The radiotoxic virulence or the determination of the speed with 

 which the toxic doses of the different rays produce lethal effects has 

 been made by computing the brevity of time required to produce 

 lethal effect for a standard radiotoxic quotum exceeding the lethal 

 radiotoxic threshold. 



A number of exposures in which many lethal wave length regions 

 appeared were selected for determination of the radiotoxic virulence. 

 The first appearances of these lethal regions were dispersed over 

 long periods. The radiotoxic quotum, or the amount of radiotoxicity 

 applied, is apparently proportional to (i) the exposure time, (2) the 

 intensity of the ray, and (3) the radiotoxic spectral sensitivity. The 

 radiotoxic virulence is, however, inversely proportional to (i) the 

 radiotoxic quotum applied, and (2) the reaction time. Determinations 

 of the virulence values for radiotoxic regions of 20 wave lengths 

 with exposure times varying from 2 to 32 minutes were made as 

 shown by the following example, which includes the computations 

 for the wave length 2463 A. 



r Radio- 1 

 Exposure f„(en- toxic C Radio- "^ 



time, X j|(y >--| spectral f — -\ toxic |-xDays= Product Reciprocals 

 Plate min. " ■ sensi- | I quotum J proportional 



A no. I tivity J to virulence 



2463 42 t6 4.16 3.00 199.8 3 599.4 .00167 



46 16 4.29 3.00 205.8 4 825.6 .00121 



47 16 4.29 3.00 205.8 4 825.6 .00121 



46 20 4.29 3.00 257.4 2 514.8 .00194 



47 20 4.29 3.00 257.4 4 1,029.6 .00097 

 42 32 4- 16 300 399-3 2 798.6 .00125 



The virulence values for each exposure were then plotted against 

 the wave lengths. 



As the curve resulting from the 32-minute exposure on plate 42 

 included all 20 of the radiotoxic regions and appeared typical, it was 

 chosen as a standard. The 12 other curves were then reduced to the 

 same scale as the standard, 42 (32), in the following manner. First, 

 each value of 42 (32) was divided by each corresponding value of 

 the 12 other exposures. For example, the computations for 2463 A 

 would be : 



Plate no 4-' 42 46 47 46 47 



Exposure time 32 16 t6 16 20 20 



2463 A i.oo 0.75 1.03 1.03 0.64 1.29 



