_Light_ acts as an inhibitory or even germicidal agent. This fact was first established by Downes and Blunt in a memoir to the Royal Society in 1877. They found by exposing cultures to different degrees of sunlight that thus the growth of the culture was partially or entirely prevented, being most damaged by the direct rays of the sun, although diffuse daylight acted prejudicially. Further, these same investigators proved that of the rays of the spectrum which acted inimically the blue and violet rays acted most bactericidally, next to the blue being the red and orange-red rays. The action of light, they explain, is due to the gradual oxidation which is induced by the sun's rays in the presence of oxygen. Duclaux, who worked at this question at a later date, concluded that the degree of resistance to the bactericidal influence of light which some bacteria possess might be due to difference in species, difference in culture media, and difference in the degrees of intensity of light. Tyndall tested the growth of organisms in flasks exposed to air and light on the Alps, and found that sunlight inhibited the growth temporarily. A large number of experimenters in Europe and England have worked at this fascinating subject since 1877, and though many of their results appear contradictory, we may be satisfied to adopt the following conclusions respecting the matter: (1) Sunlight has a deleterious effect upon bacteria, and to a less extent on their spores. (2) This inimical effect can be produced by light irrespective of rise in temperature. (3) The ultra-violet rays are the most bactericidal, and the infra-red the least so, which indicates that the phenomenon is due to chemical action. (4) The presence of oxygen and moisture greatly increases this action. (5) The sunlight acts prejudicially upon the culture medium, and thereby complicates the investigation and after-growth. (6) The time occupied in the bactericidal action depends upon the heat of the sun and the intrinsic vitality of the organism. (7) With regard to the action of light upon pathogenic organisms, some results have recently been obtained with _Bacillus typhosus_. Janowski maintains that direct sunlight exerts a distinctly depressing effect on typhoid bacilli. At present more cannot be said than that sunlight and fresh air are two of the most powerful agents we possess with which to combat pathogenic germs. [Illustration: PASTEUR'S LARGE INCUBATOR FOR CULTIVATION AT ROOM TEMPERATURE]