Petri's Dishes 



249 



the most important being that of Petri, who invented special 

 dishes to be used instead of plates. 



Petri's Dishes. These are glass dishes, about 4 inches 

 in diameter and J inch deep, with accurately fitting lids. 

 They were first recommended by Petri* and greatly sim- 

 plify bacteriologic technic by dispensing with the plates and 

 plate-boxes, the moist chambers and benches, and usually 



Fig. 54. Petri dish for making plate cultures. 



with the levelling apparatus of Koch, though this is still em- 

 ployed in some laboratories, and must always be employed 

 when an even distribution of the colonies is necessary in 

 order that they can be accurately counted. 



The method of using the Petri dishes is very simple. They 

 are carefully cleaned, polished, and sterilized by hot air, care 

 being taken that they are placed in the hot-air closet right 

 side up, and after sterilization are kept covered and in that 



Fig- 55- Petri dish forceps. 



position. They should be sterilized immediately before 

 using, or if they must be kept for a time should be wrapped 

 in tissue paper and then sterilized. The tissue paper pro- 

 tects the interior from the accidental entrance of dust be- 

 tween dish and lid, keeps the dish closed, and need not be 

 removed until the last moment before using. 



Time can be saved by sterilizing the dish and cover in 

 the direct flame, instead of in the hot-air closet, special 

 forceps (Fig. 55) adapted to holding them having been 

 devised by Rosenberger.* 



* "Centralbl. f. Bakt. u. Parasitenk.," i, No. i, 1887, p. 279. 



t "Phila. Med. Jour.," Oct. 20, 1900, vol. vi, No. 16, p. 760. 



