NO. lO 



ULTRA-VIOLET LIGHT ON GREEN ALGA MEIER 



of the lethal effect, for they should be proportional to the algae killed, 

 within the limitations due to plate thickness and other possible causes. 

 If these figures are divided by the relative intensity of the lines, an 

 approximate value is obtained of the relative lethal effect of a given 

 quantity of incident energy of different wave lengths. These values 



are given in the ratio — 



While these values have been griven to two 



figures for the sake of uniformity, the significance of the quantities 

 differs greatly for different wave lengths. Values for wave lengths 



Fig. I. — Relative lethal effect of ultra-violet light on Chlorella vulgaris. The 

 ordinates are relative lethal effect in arbitrary units. The abscissae are wave- 

 lengths in Angstrom units. (Dots occur through points of less weight.) D Ex- 

 posure I, black plate; O exposure 2, black plate; X color plate. 



2,536 A. should receive almost no weight because of the changing 

 intensity due to the progressive opacity of the arc walls. Lines 2,652 A. 

 and 2.804 A., because of the fact that they are both highly lethal and 

 very intense, are rendered doubtful, since all the algae were killed over 

 a wide range so that corrections for this thin plate effect are uncertain. 

 The black plate or second plate was taken some months later with 

 the result that the wall of the arc had become so opaque that it was 

 impossible to work with line 2,536 A. and the intensity of the line 

 2,652 A. was reduced by a quarter of its original value. Two expo- 

 sures of different lengths were obtained on this plate and are shown 

 in Plate 2, Figure i. Exposure i, which lasted three hours, gives an 

 idea of the relative effects of lines 2,652 A. and 2,804 A., which are 



