ENUMERATION OF MICROORGANISMS ll 
The dilution water in definite amounts may be put into them before 
sterilization but the quantity of water has been found to be altered 
during this procedure. To obviate this difficulty, the containers may be 
plugged and sterilized empty while the water may be sterilized in a larger 
bottle. This may then be transferred to the dilution bottles with a 
sterile pipette. 
Fra. 10 —Stewart’s Counting Apparatus 
The Petri dish 1s Wluminated by obhque rays of hght coming from an incandescent hght. 
ENUMERATION OF BACTERIA 
Several methods have been devised for counting the number of 
bacteria which are present in any substance. For convenience they 
may be classified as follows: 
A. Plate cultures. 
(a) Petri dish cultures. 
(b) Frost’s microscopic plate method. 
B. Direct microscopic method. 
(a) Breed’s method. 
(6) Heemocytometer methods. 
The plate method is probably the oldest and involves the use of 
the Petri dish. Any transparent solid medium may be used in the 
Petri dish, but the dilution of sample introduced is important. Hill 
(1908) and Breed (1916) have both shown that over-crowded plates 
