1894.] on the Action of Light on Bacteria and Fungi. 267 



By thus using various coloured solutions capable of cutting off 

 certain rays of light — their behaviour in this respect being carefully 

 determined by spectroscopic examination — in the one cell of the 

 double screen, and water in the other, I was enabled to show very 

 clearly the comparative action of the coloured light and the unde- 

 composed light passing through water, and to establish the following 

 conclusions. 



There is no perceptible bactericidal action behind any screen 

 which cuts off the blue-violei rays, whereas the action is the more 

 pronounced the more these rays are transmitted. It matters not how 

 much of the red-orange and yellow end of the spectrum is trans- 

 mitted, so long as the blue-violet end is cut off by the screen the 

 spores are not killed at the ordinary temperatures. " Results entirely 

 confirmatory of these were also obtained with glass screens, although 

 I now lay less stress on these, because they are in some respects less 

 satisfactory for exact quantitative results, owing to the variations 

 which glass screens show as regards their capacities for absorption 

 and radiation.* 



[Photographs of exposures behind other screens.] 



Numerous experiments were also made, with screens and without, 

 to test a number of other points of discussion which arose during the 

 progress of the investigation — e. g. how long an exposure was 

 necessary to kill all the spores ; how far temperature aids or combats 

 the destructive effects of the blue rays ; and so on. In these cases 

 screens of quartz, alum, quinine, sesculin, rock-salt and various other 

 substances were used, and a number of interesting facts obtained as 

 to details. 



During the course of the inquiry as to the time necessary, it was 

 found to be useful to photograph the plate after exposure in order to 

 record the gradual process of " development of the image " — as the 

 photographer would term it — as the spores on the non-illuminated 

 parts of the agar film germinated out and grew into colonies, the 

 opacity of which showed up the clear figure of the exposed area 

 by contrast: and some suggestive facts were elucidated by these 

 experiments. 



On unwrapping the exposed plate nothing is visible at first, as 

 already explained, but after twelve to twenty-four hours or so at a 

 suitable temperature in the dark, the spores on the non-illuminated 

 parts of the plate gradually produce a faint grey opacity around the 

 exposed area, which is so far a clear ill-defined patch. In the course 

 of another twelve to twenty-four hours or so the outlines of the letter 

 or other shaped area begin to show more definitely, and the figure can 

 be recognised, and later on it becomes sharper and sharper as the 

 contrast between the more and more opaque and dense bacterial 



* I am glad to take this opportunity of thanking Mr. G. W. Walker, of the 

 Royal College of Science, for the trouhle he has taken recently in physically 

 testing a series of these glass screens for me. 



T 2 



