CALCULATIONS 023 
counts, respectively. The cight large squares, A, A, etc., are the squares 
used for yeast and spore counts. Each of these squares has twenty-five 
of the small squares. The sum of the organisms counted in the eight 
squares marked A, A, etc., is the number in 1/60 c.mm. if a dilution 
of one part of product to two parts of water is used. 
Yeast and Spore Count. The ruled square on the slide is 1 mm. on 
each side and the cell is 1/10 mm.deep. The volume of the ruled part 
is, therefore 1/10 cmm. The ruled area is divided into sixteen large 
squares and the number of organisms is counted in eight of these, which 
is equivalent to 1/2 of 1/10 c.mm., or 1/20 c.mm. If adilution of one 
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Fria. 80.—Diagram of Thoma Rulings. (After Howard, 1917.) 
One millimeter divided by lines into twenty spaces in each direction each space equalling gy mm. 
To facilitate counting, every fifth space 1» subdivided by a line through the middle. 
part of the product to two parts of water is used 1/3 of 1/20 c.mm., or 
1/60 c.mm. as representing the actual amount of original stock in 
which organisms are counted, is obtained. 
Bacterial Count. The rectangles, B, B, etc., each including five of 
the smallest squares, represent the areas used in making the bacterial 
count. Similar rectangles of equal area might be selected, the object 
being to count five such areas well distributed over the ruled portion 
of the slide. The average number of bacteria counted on five rectangles, 
such as B, B, etc., multiplied by 2.4 million, equals the number of 
