Results shall be expressed in the number of Standard Units of each kind of micro-organism per cc. and also the total number of standard units of all kinds per cc. The general directions as to significant figures given under Turbidity shall apply also to the microscopical examination. _Caution._—Many micro-organisms, especially some of those causing odors, are so fragile that they are broken up in filtration, especially if the agitation of the filtrate is too vigorous. A direct examination of a fresh sample is therefore a useful supplementary procedure. For the same reason the concentrate should not stand long before examination. Also some organisms are carried by specific gravity to the top of the cell which should be scrutinized as well as the bottom layer each time. It is always better to examine the micro-organisms in the field when possible, and for this purpose the sling filter has been devised consisting of a metal funnel slung to a pivoted handle, with a disk of wire gauze in the detachable lower end to support the sand. Filtration is hastened by imparting a whirling motion to the whole and utilizing the centrifugal force thus generated. [Illustration: THE OCULAR MICROMETER.] MICROSCOPICAL BIBLIOGRAPHY. _a._ KEAN, A. L. A new method for the microscopical examination of water: _Science_, Vol. 13, p. 132, 1889; _Eng. News_, pp. 21, 276, 1889. _b._ SEDGWICK, W. T. Recent progress in biological water analysis: _J. N. E. Water Works Assoc._, Vol. 4, pp. 50–64, 1889. _c._ ——. A report of the biological work of the Lawrence Experiment Station: _Examinations by the State Board of Health of water supplies of Mass., 1887–90_, pt. 2, Purification of sewage and water, pp. 793–862, 1890. _d._ PARKER, G. H. Report upon the organisms, excepting the bacteria found in the waters of the State: _Examinations by the State Board of Health of water supplies of Mass., 1887–90_, pt. 1, Examination of water supplies, pp. 579–620, 1890. _e._ RAFTER, G. W. The microscopical examination of potable water, D. Van Nostrand Co., New York, 1910. (Contains bibliography.) _f._ CALKINS, G. N. The microscopical examination of water: _Report Mass. State Board of Health_, pp. 397–421, 1892. _g._ JACKSON, D. D. On an improvement in the Sedgwick-Rafter method for the microscopical examination of drinking water: _Tech. Quart._, Vol. 9, pp. 271–4, 1896. _h._ WHIPPLE, G. C. Experience with the Sedgwick-Rafter method at the Biological Laboratory of the Boston Water Works: _Tech. Quart._, Vol. 9, pp. 275–9, 1896. _i._ ——. Microscopy of drinking water, 3d ed., John Wiley & Sons, New York, 1914. (Contains bibliography.) BACTERIOLOGICAL EXAMINATION. I. APPARATUS.