12 BACTERIOLOGICAL APPARATUS 
will not give accurate results. Breed, in studying plates made from 
milk, found that those having more than thirty and less than four 
hundred colonies gave satisfactory results. 
Frost’s microscopic plate method and Breed’s microscopic method 
have been described in the Chapter on Milk. These methods were 
* 
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Fia. 11.—Hamocytometer with Pipettes in Case. 
originally divised for milk but undoubtedly could be adapted to other 
food substances. 
Hemocytometer. The hemocytometer, which is used for counting 
leucocytes and erythrocytes, is now. applied to the enumeration of 
bacteria in very heavily populated materials. 
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Fic. 12.—Showing Hzmocytometer Chamber. 
The instrument consists of a glass slide upon which is fastened pieces 
of glass in such a way as to form a chamber. ‘The outer glass is square 
with a round hole in the center. In this is put a round piece of glass 
of smaller diameter so that there is a shelf, moat and disk formed. 
The upper surface of the disk.is ruled into squares. These rulings are 
of various kinds as indicated in Fig. 11, the Thoma ruling being as com- 
