The term “Microscopic Organisms” shall include all organisms microscopic or barely visible to the naked eye, with the exception of the bacteria. It includes the diatomaceae, chlorophyceae, cyanophyceae, fungi, protozoa, rotifera, crustacea, bryophyta, and spongidae found in water. Fragments of organic matter, silt, mineral matter, zoöglea, etc., shall be considered as amorphous matter. The recording of amorphous matter usually serves no useful purpose and shall not be considered a part of the standard method. _Apparatus._—1. A cylindrical funnel about two inches in diameter at the top, with a straight side for nine inches, narrowed over a distance of three inches to a bore of one-half inch in diameter, and terminating in a straight portion of this diameter two and one-half inches in length. The capacity of this funnel is 500 cc. It shall be provided at the bottom with a tightly fitting rubber stopper with a single perforation and a disk of silk bolting cloth over the hole about three eighths of an inch in diameter. 2. A counting cell consisting of a brass rim closely cemented to a plate of optical glass. The shape and size of this cell are not essential but its depth shall be one millimeter. A convenient capacity is about one cubic centimeter. 3. An ocular micrometer ruled as follows: The ocular micrometer is commonly of such a size that with a 16 mm. objective and a suitable tube length, the largest square cuts off one square millimeter on the stage. _Procedure._—Filter 250 cc. of the water (more or less according to the clearness of the sample) through a one-half inch layer of quartz sand (washed and screened between 60 and 120 mesh sieves) supported by the disk of bolting cloth and rubber stopper at the bottom of the funnel. Suction may be applied to hasten the filtration. Remove the stopper and catch the plug of sand and its entrained organisms in a small beaker or test tube, washing down the inside of the funnel into the beaker with 5 cc. of clean (preferably distilled) water. Agitate the mixture of sand, water, and organisms to detach the latter from the sand grains, and quickly decant the water and the organisms in suspension to a test tube. If desired the sand may then be again washed with 5 cc. water and the wash water added to the first portion. Cover the cell partially with a cover glass, and by means of a pipette run the concentrate under the cover glass until the cell is completely filled. Cover and place on the microscope stage in a horizontal position for examination. Count the organisms in twenty fields, i. e., twenty cubic millimeters, estimating their areas in terms of Standard Units. _The Standard Unit is the smallest square in the ocular micrometer, and represents an area 20µ × 20µ, or 400 square microns on the stage._