BACTERIOLOGICAL INVESTIGATION OF WATER. 199 



onies on the plate according to the degree of dilution of the 

 water, it is easy to arrive at the number of bacteria in a cubic 

 centimeter of the water. Thus if the average number of col- 

 onies per square is 15, and there are 100 squares on the plate, 

 and the amount of water used in making the plate was 0.10 

 c.c., then 15 X 100 X 10 equals 15,000, which represents 

 the number of bacteria present in 1 c.c. of the water. 



Petri dishes may be used instead of plates, and for the 

 counting of colonies on the same special means have been 



6 8 



Fakes' apparatus for counting colonies (reduced one-third). 



devised. That of Fakes' apparatus (Fig. 71) is a cheap and 

 convenient one. It consists of a sheet of paper on which is 

 printed a black disc ruled with white lines. The Petri dish 

 is placed centrally upon this paper, and the colonies between 

 the white lines are counted, the whole circle being divided 

 into sixteen equal segments, as seen in the figure. 



