The other apparatus recommended consists of a 50 cc graduated cylinder, slides, and cover-glasses.
Since the counting chamber has been used extensively in blood examination and in yeast work, a brief description of the technique as followed in the latter may serve to give a better understanding of its limitations. First, in the preparation of the sample, the cylinder and flasks for mixing, and the pipette must be absolutely clean. The liquid to be examined is shaken thoroughly and then the measured sample withdrawn as quickly as possible to prevent the cells from settling and diluted with weak sulphuric acid (about 10 per cent), which prevents any further development of cells, and also aids both in the separation of the cells from one another and in their suspension—the latter factor being important when only a single drop is taken for examination. When counting blood cells, a normal or other salt solution is used so as to have the specific gravity of the diluent approximately that of the blood serum. The dilution is made as low as possible, since the number obtained in the count has to be multiplied by the dilution co-efficient, and any errors made are increased proportionately. A slight error when multiplied by the factor 4,000,000, the unit for each square, becomes very large in the total. The sample is shaken very thoroughly after the diluent is added, a drop of the liquid taken by means of a pipette, placed in the center of the counting chamber, and the cover-glass put in place. The withdrawal of the pipette and the transference of the drop to the chamber are done as quickly as possible to prevent the cells from sinking. The determination of the number of blood corpuscles, yeasts, or other cells in one cubic centimeter, the unit of volume generally used, will depend upon the average found in a number of squares. The number of squares to be counted is determined by making counts until a constant average is obtained, for if a true average is not obtained, the counting, naturally, is of no value. If the mounts do not show uniformity in the field, they are repeated.
In using the counting chamber for counting yeast cells and blood corpuscles, for which it was originally devised, the bodies to be examined are fairly large, well defined, and suspended in a fairly clear liquid, usually of rather high specific gravity. Even with these favorable conditions, the work must be done by observing the most careful technique in order to get relative results, which will be of value, and they are absolutely useless if any detail has been slighted or neglected. In attempting to adapt the method to food products, very different conditions are encountered—conditions which are opposed to obtaining accurate results. Food products, like ketchup, consist of a mixture of solids and liquids in which are various forms of organisms, the latter in varying condition, due to their environment and treatment, as well as to stages of disorganization.