1894.] on the Action of Light on Bacteria and Fungi. 271 



glass is interposed, the effect extends far beyond the visible spectrum 

 into the ultra-violet, to the right; but, when even a thin sheet of 

 glass is interposed it obstructs these rays, and the effect is like that 

 of the solar spectrum, as the photograph shows. 



(2) It shows even more clearly than before that the infra-red, 

 red, orange-yellow and green are without effect, and that the effect 

 weakens as we pass beyond the visible violet, and 



(3) That the maximum bactericidal effect is due to the blue violet 

 rays, for the bellying out of the cleared region about there is due to 

 the action of these rays reflected from the interior of the plate, show- 

 ing that here alone are the reflected rays powerful enough to act just 

 beyond the edges of the area. 



It is also worth notice how slight an obstacle suffices to cut off 

 the ultra-violet rays, for the two small protuberances visible in the 

 lower photograph are merely clue to two little drops of the Canada 

 balsam used for cementing the quartz to the glass having oozed out 

 and overflowed beyond the edge of the slot — nevertheless, this 

 sufficed to block out the rays there. 



These experiments will suffice to convince you that the blue- 

 violet end of the spectrum is the effective one. They compel to the 

 conclusion that the more I can expose the spores, &c, to the 

 unobstructed rays, the more rapidly are the germs destroyed, and 

 they show very clearly how fallacious may be the results where glass 

 has to be employed throughout — a fact which will be impressed yet 

 more upon us as I proceed. 



There is another thought which arises here, and that is, what a 

 poor chance such germs as these spores of fungi, yeasts, bacteria, &c, 

 must have if they were not shrouded from the solar rays by the 

 atmosphere ! This reflection is not without its significance towards 

 certain wild hypotheses as to the origin of life on our planet. 



A more practical outcome of these results with the electric arc 

 suggests itself in the probable efficiency of the naked arc light as a 

 disinfecting agent in hospital wards, railway and other carriages, &c, 

 as I have pointed out elsewhere; and we cannot overlook their 

 significance in connection with such experiments as those of the late 

 Sir William Siemens on the application of the electric light in 

 horticulture, Ac. 



If now we look at these extremely suggestive experimental 

 results from another point of view, it is obvious that these incubated 

 plates showing the contrast effects between dead and living bacteria, 

 or bacteria partly killed, as the case may be, and according as they 

 have been exposed to light or not, or exposed to certain rays or to 

 others, or exposed for longer or shorter times, and so on, are really 

 photographs ; but they are photographs where the sensitive agent is a 

 living organism, such as a bacterium, in place of a merely chemical 

 substance, such as a silver salt suspended in a medium which acts as 

 a carrier — here gelatine or agar, in place of the prepared gelatine to 

 the photographer. 



