1894.] on the Action of Light on Bacteria and Fungi. 261 



Meanwhile, other questions were being asked by those who were 

 not satisfied with this explanation, or in the ordinary course of 

 experimental inquiry into the subject. 



It had already been decided that Downes' and Blunt's experi- 

 ments could not be accepted as conclusive, because they were not 

 conducted with pure cultures, but with mixed infections of various 

 bacteria, the growth or non-growth of which was decided merely by 

 inspection of the turbidity or otherwise of the broth. Duclaux was 

 one of the first to use pure cultures in 1885, and Arloing about the 

 same period showed that some injurious effects on the spores could 

 be induced by artificial light, such as strong gas-light, though it did 

 not kill the bacteria. 



Then Geissler in 1892 came to the conclusion that the light from 

 a powerful electric arc-lamp has some retarding action on the typhoid 

 bacillus, though it was much feebler than direct sunlight, a result 

 confirmed later by Chmelewsky, thus making it probable that the 

 action of light on these organisms is merely dependent on the kind or 

 intensity of the rays and not on the source of the light. 



But the most interesting controversy was that which had mean- 

 time arisen concerning which of the various rays of the spectrum 

 are the really effective — or the most effective ones. 



That the various rays of the solar spectrum act differently on 

 plants, has long been an established fact in Botanical Science, and 

 the experiments of Famintzin, Batalin, Sachs, Pfeffer, Paul Schmidt, 

 Naegeli, Pringsheim, Elfving and others have put beyond doubt 

 that it is of material importance to an ordinary plant not only 

 whether it is excluded from or exposed to light, but also whether the 

 light to which it is exposed contains the normal proportions of the 

 various rays known to us in the visible and invisible spectrum, or is 

 deficient in some of these. 



To mention one or two illustrative cases only. The highly 

 refrangible rays in the blue-violet region of the spectrum have a 

 powerful effect on the processes of growth proper in a plant, 

 and in heliotropic curvatures of the growing parts, whereas the 

 process of carbon-dioxide assimilation is chiefly dependent on the 

 less refrangible red-orange rays at the other end of the spectrum. 

 Again, the infra-red rays, if only in virtue of their thermic effects, 

 are known to be of importance in growth, though in quite a different 

 way from the rays above mentioned ; while Sachs has given reasons 

 for believing that the invisible ultra-violet rays at the other extreme 

 of the spectrum have quite different effects agaiu in the development 

 of coloured flowers, and so on. 



It was natural, therefore, to raise the question whether all, or 

 only some, and if so which of the rays composing the solar light are 

 effective in retarding and killing bacteria — especially when we 

 remember that bacteria are plants — or at any rate that many of the 

 organisms so denominated belong to the vegetable kingdom. 



Two methods have been employed for the purpose of answering 



