182 BACTERIOLOGY. 



Wolffhugel's Counting Appaeatus. — This ap- 

 paratus (Fig. 27) consists of a flat wooden stand, the 



Fig. 27. 



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. 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. 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 

 tiie 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 facilitated by the use of a 

 small hand-lens. 



