622 APPLICATION OF METHODS OF BACTERIOLOGY 



plate is moved about until it rests under the centre of the 

 area occupied by the ruled lines. The number of colonies 

 in each square centimeter 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 indicated, 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. 108.) 



Fig. 108 



Lens for counting colonies. 



Several useful modifications of the apparatus of Wolff- 

 hiigel have been introduced. The most important is that 

 of Lafar.i 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. 



' Centralblatt fiir Bakteriologie und Parasitenkunde, 1891, Bd. xv, S. 331, 



