BACTERIOLOGICAL STUDY OF WATER. 635 



the calculation, 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 appa- 

 ratus (Fig. 93) consists of a flat wooden stand, the 

 centre of which is 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 



FIG. 93. 



Wolffhugel's apparatus for counting colonies. 



upon it. When the gelatin plate containing the colonies 

 luis been placed upon this background of glass it is 

 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 



