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 



FIG. 29. 



WolffhiigePs 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. 



