70- PRACTICAL BACTERIOLOGY. 



coming from it should be sufficiently miaute. There 

 are two reasons for this. First, the hanging drop, 

 although it must be as flat as possible, must never be 

 so large as to touch the sides or bottom of the depres- 

 sion, for iu that case the danger of impurities reaching 

 the little cultivation would be much increased, and 

 also the amount of air left in the depression would 

 be too small; but, above all, because the develop- 

 ment of the bacteria in deep drops could not be 

 observed through powerful lenses which have short 

 focal lengths. The special purposes for which hanging 

 drop cultures are used will be explained in a later 

 chapter. 



Potato cultivations are always stroke cultivations ; 

 the material containing bacteria is taken with the 

 sterilised platinum wire, and a stroke is made on the 

 slice of potato (the preparation of which was described 

 in Chapter II.), after which the dish is immediately 

 covered up. In this case also it is a good thing to keep 

 the cultivation dishes under sterilised bell-glasses. 



