CHAPTER III 

 ' INFLUENCE OF LIGHT AND OTHER RAYS 



.Microorganisms in their natural environment are temporarily, but 

 not usually exposed to light. The organisms of decay, living in soil, in 

 foods, in the intestines of animals, will only occasionally come in con- 

 tact with the direct rays of the sun. Water bacteria and the organisms 

 on the surface of plants and animals are more commonly exposed to the 

 sun. 



Fig. ioj. — These plates were heavily inoculated with B. colt ^nd B. prodigiosus 

 respectively and then were exposed, bottom side up, to the direct rays of the sun, 

 for four hours. On the instant of exposure, a figure O cut from black paper was 

 pasted to the plate shading the bacteria underneath. After one, two and three hours 

 the corresponding figures vvere pasted to the plates. The above picture was taken 24 

 hours after exposure, proving that three or four hours of direct sunlight weaken and 

 and may even kill bacteria. B. prodigiosus proved more sensitive than B. coli. 

 (Original.) 



The influence of light varies with its intensity. Direct sunlight 

 has a very harmful eSect upon microorganisms. Most bacteria are 

 killed by direct sunUght in a few hours; the time depends upon the 

 organism as well as upon the intensity of hght; this again varies with 



