The investigation to which the following communication relates was undertaken by us with the yiew of ascertaining, first, whether light could be shown to exert any appreciable influence, favourable or the reverse, upon the development of Bacteria and other organisms in certain of those solutions which afford a suitable medium for their appearance and increase. We feel, justified in thus presenting our earlier researches and the conclusions drawn from them, by considering that every fact, however small, which tends to throw light upon the life-history of these organisms is of importance as bearing upon questions of the highest moment and most varied interest.