102 ATLAS OF BACTERIOLOGY. 



are less pathogenic, etc. It is only after repeated 

 transference to fresh nutrient media in the dark that 

 they regain their old power. When the action of 

 light is still more prolonged the micro-organisms die. 

 In order to test the sensitiveness to light it is best, 

 according to H. Buchner, to expose to diffuse light 

 or to sunlight densely crowded plates of gelatin or 

 agar, a black paper cross being pasted on the light 

 side. In order to exclude the action of heat the light 

 may first be passed through a layer of water or 

 alum a few centimeters in thickness. After exposure 

 to the light for one-half, one, one and a half, two 

 hours, etc. , the plates are placed in the dark and it is 

 noted whether the bacteria develop only at the loca- 

 tion of the cross. When all the colonies which were 

 illuminated have perished, we find a sharply defined 

 cross, formed of cultures in a light field. 



During March, July, and August bacteria putidum 

 and prodigiosum are killed in one-half hour by direct 

 sunlight. In November, at the end of one and a half 

 hours, their power of producing pigment and tri- 

 methylamin is interfered with materially, they grow 

 slowly, and bacterium prodigiosum liquefies poorly. 

 The organisms died in one and a half and two and a 

 half hours. 



In diffuse daylight, inhibition of development oc- 

 curs in the spring and summer in three and a half 

 hours, in winter in four and a half hours; death 

 occurs in from five to six hours. The electric arc 

 light, of 900 candle power, inhibited development in 

 five hours, and killed the germs in eight hours. 

 Bacterium coli, bacterium typhi, and bacillus anthra- 

 cis reacted in a similar manner. 



