First add a few mu L of a CaCl_2 solution to the cell suspension. The stoc CaCl_2 500 mM. You need to take some of the cell suspension and some of the stock CaCl_2 solution and put in a microcentrifuge tube to bring the final concentration of calcium ions to 2 mM. Calculate what you need to do to dilute the stock solution for a final concentration of 2 mM. Mix the cells with the CaCl_2 solution in the microcentrifuge tube, then place 12 mu L of this mixture on the hemocytometer, then add the cover slip, and start counting.