BACTERIOLOGICAL STUDY OF WATER 621 



Note. — Prepare a plate; calculate the number of colonies 

 upon it by this latter method. Now repeat the calcula- 

 tion, making the average from another set of squares. Now 

 actually count the entire number of colonies on the plate. 

 Compare the results. 



For facilitating the counting of colonies several very 

 convenient devices exist. 



Wolffhugel's Counting-apparatus. — This apparatus (Fig. 

 107) consists of a flat wooden stand, the. centre of which is 



Fig. 107 



Wolffhugel's apparatus for counting colonies. 



cut out in such a way that either a black or white glass 

 plate may be placed in it. These form a background upon 

 which the colonies may more easily be seen when the plate 

 to be counted is placed upon it. When the gelatin plate 

 containing the colonies has been placed upon this back- 

 ground of glass it is covered by a transparent glass plate 

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

 square centimeters and subdivisions, when in position is 

 just above the colonies, without touching them. The gelatin 



