268 Professor Ward [April 27, 



growth around, and the clear area containing the invisible dead — 

 killed — spores becomes greater. 



In certain cases, especially on very hot sunny days, when the 

 light is particularly bright, it is found that the image is blurred for 

 a long time, owing to the retardation (not death) of the spores in the 

 immediate neighbourhood of the margins of the figure. The explana- 

 tion of this phenomenon seems to be that the excessively intense light 

 passing through the stencil letter and the transparent floor on which 

 the spore-laden agar-film rests, is partly reflected and slightly diffused 

 from the surface of the glass lid of the Petri dish, and this reflected 

 light is still strong enough to have some effect, and consequently the 

 spores nearest the margins of the area through which the light comes 

 are retarded, but not killed. 



[Demonstration of the gradual development of the figure during 

 incubation.] 



Another phenomenon of interest here is that a few spores here 

 and there sometimes escape being killed, even on the most exposed 

 parts of the illuminated area, and after a few days slowly grow out 

 into feeble colonies. For some time this puzzled me considerably. 

 It is clearly not evidence in support of any poison theory ; but it 

 seems very simple if we suppose these spores to have been so 

 disposed that a number of other spores, vertically situated between 

 them and the incident rays of light, sheltered them more or less 

 completely — a disposition quite possible when we remember the 

 millions of spores present in the film — and so saved them from 

 complete destruction. And all the evidence points to this being the 

 right explanation. 



It only remains to be added here that all the effects already 

 referred to can be got with the electric arc-light, instead of with 

 solar light, but there are some difficulties in employing this source 

 of radiation which may easily lead to the erroneous conclusion that 

 the electric light is less efficacious as a bactericidal agent than sun- 

 light. The principal of these is that, as we shall see, glass in any 

 form is powerfully obstructive to the rays required. 



It occurred to me at an early stage in this investigation — in fact, 

 my earlier efforts were especially directed to this end — that the most 

 direct answer to the question, What rays are the really effective 

 ones ? ought to be best obtainable by shining the spectrum itself 

 directly on the film of spores, and making the latter record the effects 

 themselves, by their subsequent behaviour, according as red, yellow, 

 blue, &c, rays fell on them. For, obviously, if the spores are killed 

 by blue- violet rays, and not injured by red and yellow ones, those 

 spores which lay in the blue, &c, of the spectrum ought to die, and 

 the agar remain clear, while those in the red, &c, ought to grow and 

 render the agar opaque, and so on, and I ought to obtain a photo- 

 graph, in living and dead bacteria, of the spectrum itself ! 



It is too long a story to go over all the difficulties and disappoint- 

 ments incident to the preliminary trials; but the ultimate success 



