88 
curacy in concentrating the sample it is 
best to use a short pipette [Fig. 7]. The 
sand [Fig. 3] used in the filter should be 
perfectly clean and of such size that its 
grains will pass through a sieve having 60 
meshes to the inch, but not through one 
having 100 meshes. Crushed quartz makes 
the best filtering material and should be 
The dises of bolt- 
ing cloth [Fig. 6] may be easily cut out 
used when obtainable. 
with a wad cutter. The filtered water 
may be used for concentrating the organ- 
isms, or it is possible to employ preserva- 
tive fluids in case the microscopical ex- 
amination must be deferred or it is desired 
to keep the specimens. The cell [Fig. 8] 
SCIENCE. 
“{N.S. Vou. VI. No. 133. 
for holding the concentrated fluid may be 
made hy cementing a brass rim to an or- 
dinary glass slip. It should be 50 mm. 
long, 20 wide and 1 mm. deep, thus hold- 
ing just 1 ¢.c. and having a superficial area 
of 1,000 sq. mm. 
A very simple microscope will answer for 
this work. A large stand is too valuable 
and too heavy for the rough usage in the 
field, and a cheap, light stand with a 3” or 
2” objective and a No. 3 ocular will answer 
equally well. The ocular must be provided 
with a micrometer, so that the observer 
may count the number of organisms in one 
cu. mm. of the cell. A disc of glass ruled 
as in Fig. 9 is the best form of micrometer, 
