Direct mi
croscopic counts are performed by spreading a measured volume of
sample over a known area of a slide, counting representative microscopic fields, and
relating the averages back to the appropriate volume
-
area factors. Specially constructed
counting chambers
, such as the Petroff
-
Hauser and Levy counting chambers, simplify
the direct counting procedure because they are made with depressions in which a known
volume overlies an area that is ruled into squares. The ability to count a defined area