1890. | RAFTER—BIOLOGICAL EXAMINATION OF WATER. 39 
I consider the precise millimetre movement of the mechanical 
stage a matter of considerable importance, and indeed insist upon it as 
an integral part of the method. Without it the tendency will be to 
sometimes select squares for counting which are contiguous, while at 
other times one will pass over squares containing few or no organisms 
in a search for more prolific ones, making, in either case, an error in 
the final result. By use of the mechanical stage with a definite formula 
for passing over the slide, personal errors of this sort are eliminated, 
leaving only those which are due to irregularity of distribution of the 
organisms in the water, and by always stirring thoroughly before taking 
the portion for examination with 1 c. c. pipette this error may also 
be reduced to a small degree, provided as many as 50 squares are 
counted as the basis of the final average. Additional uniformity of 
distribution of organisms in the cell may also be obtained by stirring 
gently in the cell itself with the pointed end of the pipette, before 
floating the cover glass to place, but the precaution should always be 
taken in these stirring operations to proceed gently in order to guard 
against breaking up unnecessarily the particles of amorphous organic 
matter which are nearly always present in any sample of water in which 
algous growths and decay are taking place. 
fig2 == 
419, 2. Sechon of open ce// Showin 
Curve of surface of Flurd due 
fe Capi Mary attraction af sides. 
The definite estimation of the amorphous organic matter is a thing 
of some difficulty, and in my own use of this method I have formed a 
sort of mental standard as to the unit of area covered by one mass of the 
amorphous matter. Mr. Geo. C. Whipple, who has assisted me in some 
experimental work for the Boston Water Works, has suggested that 
this unit be made definite for all persons by taking it a fixed number 
of square microns, and for this purpose 20 microns seems to be the 
desirable unit. By careful comparison with a stage micrometer for a 
few times this unit can be firmly fixed in mind and an estimate of the 
amount of amorphous matter made with considerable precision. 
The advantage of a cell of such depth as to just contain the 
quantity taken for examination is illustrated by figure 2, which repre- 
sents the ofen cell and shows the meniscus form taken by the liquid, by 
reason of capillary attraction at the sides. This curvature is so 
