1. A method for counting a plurality of blood cells in a sample of whole blood having a volume, said method comprising the steps of: (a) depositing the sample of whole blood onto a slide; (b) employing a spreader to create a blood smear; (c) allowing the blood smear to dry on the slide; (c) measuring an absorbance or a reflectance value of light attributable to hemoglobin in a plurality of red blood cells in each of a plurality of sections of the blood smear on the slide to determine an optical density for each of the sections of the blood smear; (e) determining an optical density of the entire blood smear; (f) recording a magnified two-dimensional digital image of an area of analysis identified by the measurement in step (d) as being of suitable thickness for analysis; (g) counting a plurality of blood cells in the area of analysis; (h) determining a volume of the sample that is present in the area of analysis by calculating a ratio of the optical density of the area of analysis to the optical density of the entire blood smear and multiplying said ratio by the volume of the sample; and (i) collecting, analyzing, and storing data from the two-dimensional digital image to count the plurality of blood cells in the sample.