BACTERIOLOGICAL STUDY OF WATER. 605 



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. 95.) 



FIG. 95. 



Lens for counting colonies. 



Several useful modifications of the apparatus of Wolff- 

 hiigel have been introduced. The most important is 

 that of Lafar. 1 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 form- 

 ing the sectors are of equal size. Three of the sectors 

 are subdivided into smaller areas of equal size for con- 

 venience in counting when the colonies are very numer- 

 ous. The principles involved are similar to those of the 



1 Lafar : Ceiitralblatt fiir Bakteriologie und Parasitenkunde, 1891, 

 Bd. xv. S. 331. 



