130 MANUAL OF BACTERIOLOGY. 



The Petri dish, however, may be placed upon the glass plate and 

 the cross lines be used to assist in counting. Lafar, Pakes and 

 JefFer recommend a surface painted black, ruled with white 

 lines which represent the radii of a circle, which may be still 

 further subdivided by other lines. Many find counting easier 

 when a black surface divided into squares is employed. An 

 ordinary card with a smooth black surface divided into "squares 

 by white lines may be placed under a Petri dish and will be 

 found to serve very well.* For the mere examination of the 

 colonies no better surface can be devised than the ferrotype plate 

 used by photographers. The examination of the colonies will 

 be easier if a small hand-lens be used. Care must be taken not 

 to mistake air-bubbles or particles of dirt for colonies of bacteria. 



In any case, if possible, all the colonies in the plate should be 

 counted. The number contained within several squares may be 

 counted and the average taken; knowing the size of the squares 

 and the area of the plate, the number contained in the whole 

 plate may be calculated. Such estimations, however, are likely 

 to give results very wide of the truth. 



The plating may be done by rolling the medium after the 

 manner of Esmarch. When the number of colonies is not large 

 this may serve very well. Counting may be assisted by draw- 

 ing lines with ink on the outer surface of the test-tube. 



It has been said that a water-supply containing no more 

 than 500 bacteria per cubic centimeter is to be regarded as 

 safe, one having between 500 and 1000 is to be looked upon 

 with suspicion, and that where there are more than 1000 to the 

 cubic centimeter the water is unfit for drinking purposes. It 

 is obvious, however, that the character of the bacteria is of 

 prime importance; that pathogenic organisms may occasion- 

 ally be present, even when the number of bacteria to the cubic 

 centimeter is small. But knowing the number usually found 



* Specially ruled cards will be found after the index at the back of the book. 



