1894. J on the Action of Light on Bacteria and Fungi. 275 



in darkness and in the light may be, however, I now throw on the 

 screen the curves obtained by measuring at intervals the growth of 

 two cultures of this bacillus, one under a microscope fully exposed to 

 the light, and the other under similar conditions, but in the dark. 



[Photograph of curves shown.] 

 The temperatures recorded by the thermometers are given also ; but 

 I do not intend to go into this matter here, as I have a large number 

 of records bearing on the matter — which is, moreover, an extremely 

 complex one. I wish you to understand, however, that the enormous 

 difference between these two curves is principally due to the differ- 

 ences in illumination, and that, as you see, the bacillus in the light 

 took as long to grow 200 units of length as that in the dark did to 

 grow 560 of the same units — in other words, the light retards the 

 growth even of the actively vegetating bacillus, a fact quite in 

 accordance with what we know of the growth of other plants. 

 Moreover, I find the curves of growth differ when the bacilli are 

 examined growing behind coloured screens, and in just the same way 

 as we should expect from the foregoing. 



The curves now shown were obtained by measuring the growth of 

 [Photograph of curves of growth in red and blue light.] 

 two cultures in the light, one behind a blue glass, the other behind a 

 red one, and alum screens were employed to diminish the heating 

 effect. Here, where bright sunlight was used, it took the bacillus in 

 blue light the same time to grow 50 units of length that the one in 

 red light required to grow over 1200 units. 



And a large number of similar experiments all agree in pointing 

 to the same end — the light acts as a retarding agent on the growth 

 of the vegetative bacillus, as well as injuriously on the germinating 

 power of the spores. In both cases it is the blue-violet rays which 

 are effective, and if these rays are sufficiently intense, or act for a 

 sufficiently long period, they kill the organism in the manner we 

 have already seen. 



Undoubtedly the most interesting results have been some of those 

 obtained with light of relatively low intensity, acting on the spores 

 themselves. First let me show you the curves of growth of three 

 cultures of a bacillus from the Thames which I have been studying 

 for some time, in which the spores had been dried by placing them at 

 80° C. in a hot oven, and allowing the temperature to fall slowly — it 

 took two hours to cool to 30° C, as the best proof I can give you 

 that these roasting hot temperatures do not injure the life of the spore, 

 aud that the results to be described are in no way due to temperature. 

 [Curves of growth of spores dried at high temperatures.] 



As the curves show, these spores germinate as rapidly and well as 

 if merely ripened at the ordinary temperature and at once sown. 



I may also add that the spores of the bacillus I am showing you 

 will endure the broth or water, in which they are suspended, being 

 raised to boiling point for a minute, and may be kept at 55° to 60° C, 

 and even higher temperatures, for many hours without apparent 



