504 



BACTERIOLOGY. 



relation to each other. To use this apparatus (Fig. 

 101) the Petri dish is placed centrally upon it, the 

 cover of the dish is removed, and the colonies are 

 counted as they lie over the spaces bounded by the 

 white lines on the black disk beneath. When the 

 plate is centred over the black disk the portion lying 

 over one sector is exactly one-sixteenth of the whole 

 plate. 



Esmarch's Counter. — Esmarch has devised a 

 counter (Fig. 102) for estimating the number of colo- 

 nies present when they are upon a cylindrical surface, 

 as when in rolled tubes. The principles and methods 

 of estimation are practically the same as those given for 

 Wolff hiigel's apparatus. 



Fig. 102. 



Esmarch's apparatus for counting colonies in rolled tubes. 



A simpler method than by tlie use of Esmarch' s appa- 

 ratus may be employed for counting the colonies in 



