16 BACTERIOLOGICAL APPARATUS 
The new method of construction entirely avoids the cemented cell and the 
attendant danger of its loosening by the drying out of the balsam cement, 
and the possibility of the loosening of the ruled counting surface 1s also greatly 
reduced by this construction. 
The parallel form of cell first suggested by Burker and used in all Levy 
counting chambers provides a more unzform distribution of corpuscles over the 
ruled area and entirely removes the effect of atmospheric pressure upon the depth 
of the solution, a source of considerable error in the circular-form chambers. 
The parallel form of cell also greatly facilitates cleaning as compared with 
the circular type. 
Critical tests have shown the Levy rulings to be more accurate, both in the 
dimensions of the ruling and the depth of the cell, than any counting chambers 
of best European makes we have tested. All Levy counting chambers are 
Vertical longitudinal section of slide, with cover glass 
in position, showing new method of construction 
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Fia, 15.—Thoma-Levy Counting Chamber, 
guaranteed to be well within such limit of error as may be later established by 
the U. 8. Bureau of Standards for instruments of this type. 
Application of Hezmocytometer to the Enumeration of Bacteria. 
Since the samples which are used in the hemocytometer are small, 
the bacteria must be present in large numbers. A slight error in 
making dilutions will cause a large error in the determination of the 
number of bacteria per cubic centimeter since the factor is large which 
converts the reading of the microscope into terms of cubic centimeters. 
It is also an easy matter to confuse organic matter with bacteria. This 
is especially true in the applications of this apparatus to the enumera- 
tion of bacteria in feces and tomato products. These are taken up in 
other places. 
Cleaning of Haemocytometer Apparatus. This is one of the most 
important essentials in the use of the apparatus. Pipettes should 
be cleaned by attaching them to a suction pump and drawing dilute 
alkali through them. After this alcohol and distilled water may be 
used. If they become stopped up a small wire may be run through, 
