Abstract This paper is in continuation of, and supplementary to, a previous communication* in which we recorded the first part of an investigation on the effect of light upon Bacteria and other organisms associated with putrefaction and decay. The chief conclusions to which those observations led us were briefly as follow:— (1.) Light is inimical to, and under favourable conditions may wholly prevent, the development of these organisms; its action on Bacteria being more energetic than upon the mycelial (and torulaceous) fungi which are prone to appear in cultivation-fluids.