136 BACTERIOLOGY. 
~ Dieudonné, in experiments upon the bacillus pro- 
digiosus, found that direct sunlight in March, July, and 
August killed these bacilli in one and a half hours; in 
November in two and a half hours. Diffuse daylight 
in March and July restrained development after three 
and a half hours’ exposure (in November four and a 
half hours), and completely destroyed vitality in from 
five to six hours. Electric arc-light’ inhibited growth 
in five hours and destroyed vitality in eight hours. 
Incandescent light inhibited growth in from seven to 
eight hours and killed in eleven hours.. Similar results 
have been obtained with B. coli, B. typhosus, and B. 
anthracis. According to Koch, the tubercle bacillus is 
killed by the action of direct sunlight in a time vary- 
ing from a few minutes to several hours, depending 
upon the thickness of the layer exposed and the season 
of the year. Diffuse daylight also had the same effect, 
although a-considerably longer time of exposure was 
required—when placed close to a window, from five to 
seven days. 
Only the ultraviolet, violet, and blue rays of the 
spectrum seem to possess bactericidal action; green 
light is very much less so, red and yellow light not at 
all. The action of light is apparently assisted by the 
admission of air; anaérobic species, like the tetanus 
bacillus, and facultative anaérobic species, such as the 
colon bacillus, are able to withstand quite well the 
action of sunlight in the absence of oxygen, the B. coli 
intense direct sunlight for four hours. 
According to Richardson and Dieudonné, the mech- 
anism of the action of light may be at least partially 
explained by the fact that in agar plates exposed to 
light for a short time (even after ten minutes’ exposure 
