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Measuring and Counting with a Light Microscope

Author(s): David R. Caprette, PhD

Counting Chamber (Hemacytometer)

A counting chamber is a specialized microscope slide with an etched surface in the form of a grid, along with supports for a cover slip, so that the space between the grid and the bottom of the cover slip is precisely calibrated. Cover slips for counting chambers are specially made and are thicker than those for conventional microscopy, since they must be heavy enough to overcome the surface tension of a drop of liquid. Counting chambers are used for determining the number of cells per unit volume.

One type of counting chamber is called a hemacytometer because it was designed for counting the formed elements (red cells, white cells, and/or platelets) in blood samples. A clean 0.4 mm thick coverslip is placed over the clean grid and a suspension is introduced into one of the V-shaped wells with a Pasteur or other type of pipet. The area under the cover slip fills by capillary action. Enough liquid should be introduced so that the mirrored surface is just covered. Overfilling can "float" the cover slip, leading to an inaccurate count. The charged counting chamber is then placed on the microscope stage and the counting grid is brought into focus at low power. It is essential to be extremely careful with higher power objectives, since the counting chamber is much thicker than a conventional slide.