In biology, a hemocytometer is used to determine the concentration of cells in a suspension. These devices were originally developed to perform blood tests --- they are a type of counting chamber that determines the number of cells per unit volume in a suspension. This can be used to count almost any type of cell, including bacteria cells. The hemocytometer is used in combination with a microscope so you can count the number of cells as they appear on a grid-like pattern.
Clean the mirrored surface inside the hemocytometer using a sheet of lens paper. Also, clean the coverslip in the same manner. When the coverslip is clean, place it inside the chamber, over the top of the counting surface.
Dilute your suspension enough so that the particles in the suspension will not overlap each other and ruin your count.
Gather an amount of the solution with a pipette and squeeze the suspension into the V-shaped well so the mirrored surface is just barely covered in the suspension.
Place the counting chamber on the microscope's stage, then adjust your microscope to low power so that the counting grid is brought into focus. Then, adjust the magnification to recognise the bacteria type you are looking at.
Count the number of bacteria cells in each small grid. You will have to develop your own counting pattern based on the size of the cells. If the cells are large, count the number of cells in the four corner squares, plus the central square.
Determine the concentration of cells in the suspension. So, let's say you counted 200 cells in the five main squares. Each square you count has a volume of 0.004mm cubed, so if you counted five squares, their combined volume is 0.02 millimetre cubed.
Divide the number of cells you counted by the total volume of the squares. So if you counted 200 particles in a 0.002 millimetre cubed volume, the concentration would be 200/0.02 which is equal to 10,000 cells per mm cubed.
Multiply the resulting volume by 1,000, since there are 1,000 cubic millimetres in one cubic centimetre which is the same as a millilitre. So, your resulting concentration would be 10 million cells per millilitre.