ix EFFECT OF SUNLIGHT ON BACTERIA 43 



(k) From this tube inoculate two other tubes in the 

 ordinary manner, and pour three plates as described in a 

 former lesson (vide p. 29). 



Keep the plates at 22 C. and examine them from day 

 to day. 



Action of Sunlight on the Vibrio Cholerse Asiatics 



To demonstrate the action of sunlight on cholera vibrios 

 take two agar-agar plates (Petal's capsules) 1 which have 

 been kept in the warm incubator for twelve to twenty-four 

 hours. 



With a sterilised brush gently paint the surface of 

 the agar-agar with a dilute young broth culture of cholera 

 vibrios. 



Over the lid of each capsule gum a piece of black paper 

 with a large figure or letter cut in it, and expose the plates 

 to the sun for four to six hours. 



Then transfer the plates to the warm incubator. 



Examine them on the next day : there will be no 

 growth (or only a limited growth) on the agar-agar 

 over those areas which were unprotected from the 

 action of the light by the black paper. 



1 For preparation of agar-agar plates see Part II. 



