Kathy,
There are two possibilities open to you (sorry, three but I assume 
superimposing a grid thru you software is one you have thought of).

Microscopically, it depends upon whether you are counting a "wet 
preparation" or a dry slide.

For the former, there are "counting chambers" which are accurately 
made and carefully calibrated and inscribed.  Thr result is they are 
of a known depth (between the slide surface and the underside of a 
coverslip (coverglass) and have, in the business part of the slide, a 
(usually) one mm square area inscribed with 10x10 divisions.
The counting chamber is "loaded" by capillary attraction - 
manually.  There are two common types:- Fuchs-Rosenthal 
(0.2mm depth) and improved Neubauer (0.1 mm deep).

Thus you know the area and the depth and hence the volume - 
simple mathematics - if there is such a thing! and you have 
gridlines to help count.  You will receive instructions on the cunning 
way to know which (mini)  squares you have counted.

The alternative for counting a "dry" preparation is an eyepiece 
graticule and slide micrometer.  The later allows you to calibrate 
the graticule - which, as the name implies, resides within the 
eyepiece (or camera tube mount) (you will find a "ledge" therein on 
which it should sit and foregive me if I am teaching grandmother 
how to suck eggs - did she really do that?)
Then its simple maths - or PC program again, remembering to 
repeat the callibration exercise for each objective you use.




Russ Allison, 
Dental School
Cardiff
Wales