36 



Drs. C. H. Browning and S. Russ. 



spectrometer ; a series of photographs was then taken of the radiation 

 transmitted through the emulsion. The bottom strip (fig. 2) shows that the 

 region of wave-lengths absorbed by the bacterial emulsion corresponds 

 almost exactly with that portion of the spectrum which has marked 

 germicidal action. "We shall return to this point later on. 



The Germicidal Action upon Various Organisms. 



The method which has been described to determine what portion of the 

 ultra-violet spectrum is responsible for germicidal action is well adapted for 

 comparative tests of the effects of these rays upon different organisms. 



(a) Bacillus coli communis and Bacillus typhosus. — To determine the 

 respective ranges of susceptibility of these organisms, emulsions from agar 

 cultures were made and spread over a surface of agar, which took the place 

 of the photographic film, as already described. A central strip, about 3 cm. 

 wide, running the whole length of the plate, was separated from the rest of 

 the agar by two glass strips a few millimetres in width. The central strip 

 was painted over with the emulsion of one of the organisms, and the rest of 

 the agar with that of the other organism. The plate was then placed in 

 position in the camera, and the length of the slit adjusted so that the 

 radiation from the arc illuminated both surfaces over which the organisms 

 were spread, one of the glass strips lying exactly along the middle of the 

 exposed area. In this way the two organisms were exposed to radiation of 

 identical character for the same time ; after a suitable exposure, the camera 

 attachment was racked down, so that the second glass strip was in the 

 centre of the exposed area, and another exposure made. The agar plate 

 was removed and incubated overnight ; the resulting growth on the plate 

 showed which organism had been affected by a greater range of the rays. 



A series of observations, ranging from 5-10-20-30 minutes, showed that 

 a lethal effect upon B. typhosus was obtained over a wider range than with 

 B. coli; this difference in range of susceptibility was small, but quite 

 definite for each of the exposures ; for an exposure of five minutes, a 

 lethal effect was obtained on the B. coli over a range of wave-lengths 

 2960-2450 A.U. ; for the B. typhosus the range was 2960-2400 A.U. ; when 

 the exposure was lengthened to 30 minutes, the lethal effect on B. coli 

 ranged from 2960 to 2200 A.U. ; and that of the B. typhosus from 

 3000 to 2100 A.U. Such small differences could hardly serve as a sure 

 method of differentiation of these organisms. 



(b) Acid-fast Bacillus {Timothy Grass B) and the Meningococcus. — The 

 ranges of susceptibility of these organisms to the rays in question was 

 compared directly with staphylococcus by the method detailed above. (In 



