38 ROCHESTER ACADEMY OF SCIENCE, [March 1o, 
contained in the 500 c. c. of water are all in the sand at lower end of 
funnel stem. The plug of wire cloth is now removed, and the sand and 
contained organisms washed with 5 c. c. of freshly filtered water, run 
from a 5 c. c. pipette, into a 5 or 6 inch test tube. The test tube is 
slightly shaken in order to wash all the organisms clear from the sand. 
The sand by reason of greater specific gravity sinks quickly to the 
bottom, leaving the organisms distributed through the water. At the 
instant of the completion of the settling of the sand the supernatant 
water is turned into another smaller test tube, leaving the clean sand at 
the bottom of the first tube. We now have the organisms from 500 c. 
c. of water concentrated into 5 c. c. in the second tube, from which 
after slight stirring, to insure uniform distribution, 1 c. c. is taken with 
aic.c. pipette and transferred to a cell 50 by 20 millimetre area, and 
exactly 1 millimetre in depth. Suchacellof course contains 1000 cubic 
millimetres, or 1c. c. The top of the metal cell is ground perfectly 
smooth and with a little practice one can float a thick cover glass to 
place without losing a drop. 
The next step is the enumeration. This is accomplished by trans- 
ferring the cell to the stage of a microscope, the eye-piece of which is 
fitted with a micrometer so ruled as to cover, with a given objective, 
and fixed tube length, a square millimetre on the stage. The microscope 
itself is fitted with a mechanical stage with millimetre movement in 
both directions ; and for this purpose I have made certain simple 
additions to the new mechanical stage of the Bausch and Lomb Optical 
Company, by means of which the desired result is obtained at slight 
expense. The count is made by beginning at one corner of the cell 
and going systematically over the area in accordance with such a formula 
as will insure the count of squares selected from every part of the slide. 
The number of squares actually counted will depend upon the degree 
of accuracy which it is desired to attain. It is obviously impossible to 
count the rooo squares composing the entire area of the slide, and the 
practical question arises as to just what multiple of tooo shall be used 
to secure a correct average. This can only be determined by trial and 
comparison upon a number of samples. In any case not less than 20 
squares should be counted, and if time will possibly permit I should 
prefer to always count at least 50. 
In order to illustrate the matter, I have prepared a table which repre- 
sents the area of the cell divided into 1000 squares. Brief inspection of 
this table will show the difficulty of obtaining true averages when only 
20 squares are counted, and exhibits clearly the value of counting 
the larger number if one cares for true averages. (See Plate 1.) 
