456 BACTERIOLOGY. 



upon it. When the gelatin plate containing the colo- 

 nies has been placed upon this background of glass, 

 it is then covered by a transparent glass plate which 

 swings on a hinge. This plate, which is ruled in square 

 centimetres and subdivisions, when in position, is just 

 above the colonies without touching them. 



The gelatin 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 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 indicated, if the number of col- 

 onies be very great a mean may be taken of the number 

 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. 93. 



Lens for counting colonies. 



In Fig. 93 is seen the form of hand-lens commonly 

 employed. 



ESMAKCH'S COUNTER. Esmarch has devised a 

 counter (Fig. 94) for estimating the number of col- 

 onies present when they are upon a cylindrical sur- 



