502 BACTERIOLOGY. 



in several (6 or 8) squares; this is to be multiplied by 

 the total number of squares occupied by the gelatin. 

 The result is an approximation of the total number of 

 colonies. 



When the colonies are quite small, as is frequently 

 the case, the counting may be rendered easier by the 

 use of a small hand-lens. 



FIG. 100. 



Lens for counting colonies. 



In Fig. 100 is seen the form of hand-lens commonly 

 employed with this apparatus. Several useful modifi- 

 cations of this apparatus have been introduced. The 

 most important is that of Lafar (Centralblatt fur Bakte- 

 rlologie und Parasitenkunde, 1891, Band xv. p. 331). 

 Lafar' s counter consists of a glass disk of the diameter 

 of ordinary size Petri dishes. It is supplied with a 

 collar or flange that fits around the bottom of the Petri 

 dish, and thus holds the counter in position. The disk 

 is ruled with concentric circles and its area divided into 

 sectors of such size that the spaces between the con- 

 centric circles and the radii forming the sectors are of 

 equal size. Three of the sectors are subdivided into 

 smaller areas of equal size for convenience in counting 

 when the colonies are very numerous. The principles 

 involved are similar to those of the preceding appara- 

 tus, but the circular form of the apparatus admits of 



