STUDY OF WATER. 435 



nies has been placed upon this background of glass, 

 it is then covered by a transparent glass plate which 

 swings on a hinge. When this plate is in position, it 

 is just above the colonies without touching them. This 

 plate is ruled in square centimetres and subdivisions. 



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. 



Where 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. 89. 



Lens for counting colonies. 



In Fig. 89 is seen the form of hand-lens commonly 

 employed. 



Esmakch's Cotjxter. — Esmarch has devised a 

 counter (Fig. 90) for estimating the number of colonies 

 present when they are upon a cylindrical surface, as 

 when in rolled tubes. The principles and methods of 

 estimation are practically the same as those given for 

 Wolffhiigel's apparatus. If the number of colonies in 

 an Esmarch tube is to be determined, a simpler method 

 than the use of his apparatus may be employed. It 

 consists in dividing the tube by lines into four or six 

 longitudinal areas which are subdivided by transverse 



