When ready to make the count, mix again thoroughly by shaking; blow two or three drops of the fluid from the pipet, wipe off its tip, and then place a small drop (the proper size can be learned only by experience) upon the disc of the counting chamber. Adjust the cover immediately. Hold it by diagonal corners above the drop of fluid so that a third corner touches the slide and rests upon the edge of the platform. Place a finger upon this corner, and, by raising the finger, allow the cover to fall quickly into place. If the cover be properly adjusted, faint concentric lines of the prismatic colors--Newton's rings--can be seen between it and the platform when the slide is viewed obliquely. They indicate that the two surfaces are in close apposition. If they do not appear at once, slight pressure upon the cover may bring them out. Failure to obtain them is usually due to dirty slide or cover--both must be perfectly clean and free from dust. The drop placed upon the disc must be of such size that, when the cover is adjusted, it nearly or quite covers the disc, and that none of it runs over into the "ditch." There should be no bubbles upon the ruled area. Allow the corpuscles to settle for a few minutes, and then examine with a low power to see that they are evenly distributed. If they are not _evenly distributed over the whole disc_, the counting chamber must be cleaned and a new drop placed in it. Probably the most satisfactory objective for counting is the special one-sixth for blood work already mentioned. To {154} understand the principle of counting, it is necessary to remember that the large square (400 small squares) represents a capacity of one-tenth of a cubic millimeter. Find the number of corpuscles in the large square, multiply by 10 to find the number in 1 c.mm. of the diluted blood, and finally, by the dilution, to find the number in 1 c.mm. of undiluted blood. Instead of actually counting all the corpuscles, it is customary to count those in only a limited number of small squares, and from this to calculate the number in the large square. In practice a convenient procedure is as follows: _With a dilution of 1:200, count the cells in 80 small squares, and to the sum add 4 ciphers; with dilution of 1:100, count 40 small squares and add 4 ciphers._ Thus, if with 1:200 dilution, 450 corpuscles were counted, the total count would be 4,500,000 per c.mm. This method is sufficiently accurate for all clinical purposes, provided the corpuscles are evenly distributed and two drops from the pipet be counted. It is convenient to count a block of 20 small squares in each corner of the large square. It distribution be even, the difference between the number of cells in any two such blocks should not exceed twenty. Corpuscles which touch the upper and left sides should be counted as if within the squares, those touching the lower and right sides, as outside, and _vice versâ_.