{163} Technic.--A larger drop of blood is required than for counting the erythrocytes, and more care in filling the pipet. Use the pipet with 11 engraved above the bulb. Suck the blood to the mark 0.5 or 1.0, and the diluting fluid to the mark 11. This gives a dilution of 1:20 or 1:10, respectively. The dilution of 1:20 is easier to make. Mix well by shaking in all directions except in the long axis of the pipet; blow out two or three drops, place a drop in the counting chamber, and adjust the cover as already described (p. 153). [Illustration: FIG. 72.--Türk ruling of counting chamber.] Examine with a low power to see that the cells are evenly distributed. Count with the two-thirds objective and a high eye-piece, or with the long-focus one-sixth and a low eye-piece. {164} A one-fourth objective will be found very satisfactory for this purpose. With the ordinary ruling of the disc, count all the leukocytes in the large square, multiply by 10 to find the number in 1 c.mm. of diluted blood, and by the dilution to find the number per c.mm. of undiluted blood. In every case at least 100 leukocytes must be counted as a basis for calculation, and it is much better to count 500. This will necessitate examination of several drops from the pipet. With the Zappert and Türk rulings a sufficient number can usually be counted in one drop, but the opportunity for error is very much greater when only one drop is examined. [Illustration: FIG. 73.--Size of field required in counting leukocytes as described in the text.] In routine work the author's modification of the "circle" method is very satisfactory: Draw out the tube of the microscope until the field of vision has a diameter equal to eight {165} times the side of a small square (Fig. 73). The area of this field closely approximates one-tenth of a square millimeter. With a dilution of 1:20, count the leukocytes in 20 such fields upon different parts of the disc without regard to the ruled lines, and to their sum add two ciphers. With dilution of 1:10, count 10 such fields, and add two ciphers. Thus, with 1:10 dilution, if 150 leukocytes were counted in 10 fields, the leukocyte count would be 15,000 per c.mm. To compensate for possible unevenness of distribution, it is best to count a row of fields horizontally and a row vertically across the disc. This method is applicable to any degree of dilution of the blood, and is simple to remember: _one always counts a number of fields equal to the number of times the blood has been diluted, and adds two ciphers_. It is frequently impossible to obtain the proper size of field with the objectives and eye-pieces at hand. In such case, place a cardboard disc with a circular opening upon the diaphragm of the eye-piece, and adjust the size of the field by drawing out the tube. The circular opening can be cut with a cork-borer.