There has been little advance in our knowledge of mechanism of light sensitivity since 1900, when Raab published the results of his experiments concerning the effects of acridine hydrochloride on paramecia. Briefly, he found that an acridine solution of 1:20,000 would kill paramecia exposed to direct sunlight in 6 minutes, and that in diffused light it took 60 minutes to kill the paramecia, while in the dark it had no lethal action whatsoever. Since Raab's publication there have been several reviews of the subject, the most recent being those of Matthews (1937) and Clare (1952). Although the pathological changes of photosensitization have been described in great detail by Hausmann (1911) and again by Levy (1926), the mechanism by which these changes are brought about has remained obscure. Hausmann thought capillary thrombi accounted for the necrotizing lesions, while Levy ascribed them to circulatory stasis. The object of his paper is to