BACTERIOLOGICAL STUDY OF WATER. 501 



WoLFFHiJGEL's COUNTING- APPARATUS. — This appa- 

 ratus (Fig. 99) 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 upon it. 



Fig. 99. 



WolffhUg^l's apparatus for counting colonies. 



When the gelatin plate containing the colonies 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 colo- 

 nies, 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 colo- 

 nies be \evy great a mean may be taken of the number 



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