1894.] on the Action of Light on Bacteria and Fungi. 269 



was so astoundingly complete and instructive that I must give some 

 account of it. 



The chief difficulty to be overcome here is the great weakening 

 of intensity of the dispersed rays of the beam of light decomposed 

 to form the spectrum ; for not only are we here distributing the 

 incidence of the rays themselves over a larger area, but they are 

 passing through lenses, prisms, &c, which absorb or reflect many of 

 them. This, of course, has to be compensated by condensing the 

 beam and exposing for longer periods ; but even then, when anything 

 like a pure spectrum is employed, the slit has to be so narrow that 

 relatively little light can be utilised. Consequently, the exposure 

 has to be much more protracted than with undecomposed light. 



Then came in another difficulty. It was found, when working 

 with the electric light, that very feeble effects could be got as 

 compared with those produced by exposure to the solar rays on a 

 clear blue clay, and the reason of this is that the blue and violet rays 

 are stopped to a large extent by the glass surfaces through which the 

 rays must pass. The effect of the glass is practically the same as the 

 effect of mist or haze, &c, in the atmosphere, which so filters out 

 the blue-violet rays, that the light of a dull day is of little effect in 

 my experiments. 



During the progress of the experiments I had the good fortune 

 to obtain the aid of Professor Oliver Lodge, who was so kind as 

 to expose a large number of my plates to a powerful electric arc 

 for me, with most satisfactory results. The difficulties referred to 

 were got over by employing quartz instead of glass — the lenses and 

 prisms were of quartz and a quartz plate was used for covering the 

 agar film with its embedded spores. 



In this manner it was possible to obtain a very pure spectrum 

 sufficiently rich in blue and violet rays to kill the spores in a few 

 hours. 



With the solar rays, I found it quite easy to obtain satisfactory 

 results in the summer, however, even with glass mirrors, lenses and 

 prisms, and exposures of five to six hours, though in winter I find 

 the exposures necessary are so long as to be almost impracticable. 



I will first describe the experiments with the solar spectrum 

 which I made at Cooper's Hill this last summer. 



The plate of agar, with spores or bacilli in it, is made exactly as 

 before, but instead of the ordinary glass lid I employ a sheet of thin 

 flat ground glass, in which a slot is cut about three inches long by 

 one inch wide. This slot is covered by either a thin sheet of glass 

 or a plate of quartz. The whole of the covered plate — except the 

 slot described — is then protected from the light by black paper and 

 tin foil, and is then laid, bottom downwards, on a box kept cold with 

 ice, in such a position that the spectrum falls vertically across the slot, 

 the coloured bands crossing the long axis of the slot at right angles. 



[Demonstration of quartz-covered plates and apparatus for 

 spectrum and method of exposure, ivc] 



