THE MICROSCOPE IN MEDICINE AND SANILALION. 1493 
Water Department of Brooklyn, and other sanitary 
authorities, is as follows: A measured quantity of water, 
half a liter or a liter, is filtered through perhaps a quarter 
of an inch of fine sand on a circle of bolting-cloth sup- 
ported by a perforated rubber stopper in a tall funnel. 
The sand with the organisms collected on it is dropped 
into a test-tube, shaken up with a small volume of water, 
and allowed to settle for a moment. The water is then 
poured off and a second portion of wash-water serves 
to remove almost all the organisms from the sand. The 
two washings mixed together are then made up to some 
convenient volume, 10 or 15 cc., and thus one has in say 
ro cc. the organisms originally present in a liter. 
One cubic centimeter of the concentrate is then placed 
in a cell, 1 mm. deep and 1000 sq. mm. in area. A special 
type of ocular micrometer is used with a square ruled 
upon it, and the lenses and draw-tube are so adjusted 
that the square shall coincide with 1 sq. mm. on the stage. 
Ten or twenty fields in different parts of the cell are 
then examined and the organisms counted, when a 
simple calculation enables one to compute the number 
present in 1 cc. of the original sample. 
By this method the kind and number of micro-organ- 
isms present may be easily determined, the cause of 
existing trouble detected, and the prospect of future diffi- 
culties predicted. The efficacy of methods of purifica- 
tion may be tested and the source of trouble located in 
the particular part of a supply where it exists. 
A few of the most serious odor-producing organisms 
are figured in Fig. 48. At 4 is Anabena, the Blue-green 
