BAOTEBIOLOQIOAL STUDY OF WATER. 603 



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

 106) 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 Countek. — Esmarch devised a counter 

 (Fig. 107) for estimating the number of colonies present 



Fig. 107. 



Esmarch's apparatus for counting colonies in rolled tubes. 



upon a cylindrical surface," as when in rolled tubes. 

 The principles and methods of estimation are practically 

 the same as those given for AVolffhiigel's apparatus. 

 A simpler method than by the use of Esmarch's 



' Copies of this apparatus are to be had of Ash & Co., 42 Southwark 

 Street, London, or of Lentz & Sons, North Eleventh Street, Philar 

 delphia. Pa. (The cost is but a few cents per copy.) 



