Those who have used the method of Leishman or the later method of Wright for estimating the phagocytic or opsonic indexes of the blood well know that it is essential to use bacterial suspensions containing uniform numbers of bacteria in order to secure uniformity of results. If the suspension used for one estimation contains twice as many bacteria as that used at another the results will vary, though not mathematically, according to the density of suspension.
The method recommended by Wright is ingenious though cumbersome and depends on the actual enumeration of the bacteria contained in the suspension, and is accomplished by mixing a given volume of the bacterial suspension with an equal volume of blood, dropping a diaphragm with a small aperture into the eyepiece of the microscope, and then counting the number of blood corpuscles and bacteria in each of a number of microscopic fields. The computation that
McFARLAND J. THE NEPHELOMETER:AN INSTRUMENT FOR ESTIMATING THE NUMBER OF BACTERIA IN SUSPENSIONS USED FOR CALCULATING THE OPSONIC INDEX AND FOR VACCINES.. JAMA. 1907;XLIX(14):1176-1178. doi:10.1001/jama.1907.25320140022001f