1897.] Biological Studies in Massachusetts. 507 
drical glass funnel upon a perforated rubber stopper, covered 
with a disc of bolting cloth, the filtration sometimes being 
hastened by the use of the aspirator. The sand with whatever 
organisms have collected upon it is then washed into a test 
tube with 5 or 10 c.c. of distilled water, thus concentrating the 
organisms. This tube is shaken, and the water decanted to a 
second tube, the sand being left behind. 1 c.c. of this fluid 
containing the concentrated organisms is next placed in a cell, 
50 x 20 mm. and 1 mm. deep, covered with a thick cover glass 
and placed under the microscope. A ruled square in the mi- 
crometer ocular covers one square millimeter on the stage, 7. e., 
one-thousandth of the area of the cell. Theorganisms seen with- 
in this square are then counted, and the cell is moved so that 
other portions come into view. Having counted a definite 
number of these squares the determination of the number of or- 
ganisms in the original sample is a matter of calculation. On 
account of the great variety in size which the different organ- 
isms present it has been found expedient to adopt a standard 
unit of size (400 square microns) in terms of which all organ- 
isms are recorded. The organisms are classified according to 
the following convenient grouping: 1. Diatomace ; 2. Chloro- 
phycee; 3. Cyanophycee; 4. Fungi; 5. Rhizopoda; 6. In- 
fusoria; 7. Rotifera; 8. Crustacea; 9. Miscellaneous. 
Occasionally microscopical examinations are made in the 
field, in order to study the sample while fresh and before the 
organisms have begun to break up. For this purpose a conve- 
nient portable outfit is provided. Another feature of the micro- 
scopical work consists of the examination of material from the 
shores and bottoms of the reservoirs, the organisms growing in 
the swamps, along the streams, and upon the walls of the 
aqueducts and distribution pipes. 
3. Bacteriological—The bacteriological work is chiefly the 
weekly determination of the number of bacteria in the water 
of the aqueducts, distribution reservoirs and service pipes by 
means of the ordinary methods of culture. More or less study 
is given to the various species observed, and a careful watch is 
kept for the presence of Coli communis, whose presence to a 
certain extent is considered an index of pollution. At some- 
