BACTERIOLOGICAL STUDY OF WATER. 601 



in each square centimetre is then counted, and the sum- 

 total of the colonies in all these areas gives the number 

 of colonies on the plate ; or, as has already been indi- 

 cated, if the number of colonies be very great, a mean 

 may be taken of the number in several (six or eight) 

 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. 105.) 



Fig. 105. 



Lens for counting colonies. 



Several useful modifications of this apparatus have 

 been introduced. The most important is that of Lafar.' 

 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 is divided into sectors of 

 such sizes that the spaces between the concentric 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 



1 Centralblatt fiir Bakteriologie und Parasitenkunde, 1891, Bd. xv. 

 S. 331. 



