624 APPLICATION OF METHODS OF BACTERIOLOGY 



though the areas of the smallest subdivisions are not of one 

 size and do not bear a constant relation to each other.' To 

 use this apparatus (Fig. 109) the Petri dish is placed cen- 

 trally 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 centered over the black disk the portion lying over one 

 sector is exactly one-sixteenth of the whole plate. 



Fig. 110 



Esmarch's apparatus for counting colonies in rolled tubes. 



Esmarch's Counter. — Esmarch devised a counter (Fig. 

 110) for estimating the number of colonies present upon 

 a cylindrical surface, as when in rolled tubes. The prin- 

 ciples and methods of estimation are practically the same 

 as those given for Wolffhiigel's apparatus. 



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

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

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



