The most widely used and most satisfactory instrument for counting the corpuscles is that of Thoma-Zeiss. The hematocrit is not to be recommended for accuracy, since in anemia, where blood counts are most important, the red cells vary greatly in size and probably also in elasticity. {151} The hematocrit is, however, useful in determining the relative volume of corpuscles and plasma, and seems to be gaining in favor. * * * * * [Illustration: FIG. 70.--Ordinary ruling of counting chamber, showing red corpuscles in left upper corner.] The Thoma-Zeiss instrument consists of two pipets for diluting the blood and a counting chamber (Fig. 69). The counting chamber is a glass slide with a square platform in the middle. In the center of the platform is a circular opening, in which is set a small circular disc in such a manner that it is surrounded by a "ditch," and that its surface is exactly one-tenth of a millimeter below the surface of the square platform. Upon this disc is ruled a square millimeter, subdivided into 400 small squares. Each fifth row of small squares has double ruling for convenience in counting (Fig. 70). A thick {152} cover-glass, ground perfectly plane, accompanies the counting chamber. Ordinary cover-glasses are of uneven surface, and should not be used with this instrument. It is evident that, when the cover-glass is in place upon the platform, there is a space exactly one-tenth of a millimeter thick between it and the disc; and that, therefore, the square millimeter ruled upon the disc forms the base of a space holding exactly one-tenth of a cubic millimeter. [Illustration: FIG. 71.--Method of drawing blood into the pipet (Boston).] Technic.--To count the red corpuscles, use the pipet with 101 engraved above the bulb. It must be clean and dry. Obtain a drop of blood as already described. Suck blood into the pipet to the mark 0.5 or 1. Should the blood go beyond the mark, draw it back by touching the tip of the pipet to a moistened handkerchief. Quickly wipe off the blood adhering to the tip, plunge it into the diluting fluid, and suck the fluid up to the mark 101, slightly rotating the pipet meanwhile. This dilutes the blood 1:200 or 1:100, according to the amount of blood taken. Except in cases of severe {153} anemia, a dilution of 1:200 is preferable. Close the ends of the pipet with the fingers, and shake vigorously until the blood and diluting fluid are well mixed. When it is not convenient to count the corpuscles at once, place a heavy rubber band around the pipet so as to close the ends, inserting a small piece of rubber-cloth or other tough, non-absorbent material if necessary to prevent the tip from punching through the rubber. It may be kept thus for twenty-four hours or longer.