ACTION OF LIGHT ON MICRO-ORGANISMS 357 



light, after passage through, a stratum of potassium 

 dichromate solution, was without effect. 



Geisler,^ working in St. Petersburg, published a paper 

 in February, 1892, on the action of light on bacteria, in 

 which he endeavours to differentiate between the effects 

 produced by the rays of the electric light and the rays 

 of the sun respectively on typhoid bacilli. As culture 

 material 10 per cent, gelatine-peptone was employed. 

 A series of six test-tubes, containing gelatine, were inocu- 

 lated with typhoid bacilli streaked over the surface. 

 Two were placed in a dark cupboard, two were exposed 

 to direct sunlight, and two were exposed to the light 

 from a 1,000-candle electric arc lamp at a distance of 

 1 metre. It was found that in the latter case three 

 hours' exposure produced a distinctly retarding effect 

 upon the typhoid growth, but that after six hours this 

 result was considerably more apparent. In the case of 

 direct sunlight, on the other hand, two hours' insolation 

 ah-eady produced a very markedly deleterious effect. 

 The powerful electric light thus haSj as would be anti- 

 cipated, a less rapid bactericidal action than direct 

 ■sunshine. 



Experiments were made to ascertain if the action of 

 light was affected by the accompanying rise in tempera- 

 ture, and for this purpose typhoid tube-cultures were 

 exposed to the direct rays of the sun and electric lamp, 

 after the latter had both been passed through a solution 

 of alum, whereby the heat rays are absorbed, permitting 

 the so-called light and chemical rays of the spectrum 

 to pass unimpeded. The following results were ob- 

 tained : — 



In the control tubes kept in the dark during from 

 two to three hours an abundant growth took place. 



^ * Zur Frage tiber die Wirkung des Lichtes auf Bakterien,' Central- 

 ilattfUr BaMeriologie, vol. xi., 1892, p. 161. 



