The germicidal action of ultra-violet radiation, and its correlation with selective absorption

A new method is here described which enables us to say definitely what portion of the ultra-violet spectrum is especially effective in germicidal action and the wave-length of the radiation at which such action practically ceases. Briefly the method consists of inoculating a gelatine* plate with micro-organisms instead of sensitising it with a silver salt. We find that when a spectrum is formed on this it produces what may be called an image, where germicidal action occurs, and this image may be rendered visible by a process of incubation, which encourages a copious growth of those organisms which have not been affected by the radiation, whereas the affected parts remain practically transparent. Such an exposed and incubated plate can be used as an ordinary negative for producing positive contact prints or, equally well, may be photographed by light reflected and scattered from the bacterial surface. The study of the action of radiation, visible and otherwise, upon micro-organisms is not a new one. The period 1894-6, associated with the work of Marshall Ward,* Wesbrook,† and D’Arcy and Hardy,‡ conferred upon such investigations an exactness which had been previously entirely lacking, both as regards knowledge of the most effective germicidal portion of the visible rays, and also with regard to the essential chemical processes associated with such action.