BACTERIOLOGICAL TECHNIQUE. 227 
some one of the four will have the required number of 
colonies. In the first tube we place an amount which we 
believe will surely contain sufficient and probably too 
many bacteria. To the second tube we add 10 per cent. 
of the amount added to the first, and to the third 10 
per cent. of the second, and to the fourth 10 per cent. 
of the third. Thus if the first contained 60,000 colo- 
nies, the second would have 6000 (Fig. 28), the third 
600, and the fourth 60. If, on the other hand, the first 
contained but 60, the second would have about 6, and 
the remaining two would probably contain none at all. 
When there are many colonies present the dishes are 
covered by a glass plate (Fig. 29), ruled in larger and 
Fie. 29. 
Wolffhiigel’s apparatus for counting colonies. 
smaller squares. With a hand lens the colonies in a 
certain number of squares are counted and then the 
number for the whole contents estimated. 
When the material to be tested is crowded with bac- 
teria it is often best to make an emulsion of a portion 
of it, and use this rather than the original substance 
for making the cultures. 
Measured quantities of the diluted material can be 
transferred most accurately through a sterilized long 
glass pipette graduated in one-hundredth cubic centi- 
metres, or, more roughly, by a platinum loop of known 
size. 
