504 The American Naturalist. [June,. 
the organisms caught by the cloth were subsequently trans- 
ferred to a small quantity of water, and placed under the micro- 
scope for identification and enumeration. In June, 1889, the 
“cloth method ” was superseded by the “sand method ” devised 
by A. L. Kean, and improved by Prof. Sedgwick. The water 
was filtered through sand instead of cloth, after which the sand 
with its collected organisms was washed into a deep cell and 
placed on the stage of the microscope. To assist in the enu- 
meration the bottom of the cell was ruled in squares. This. 
method was used until November, 1890, when the Sedgwick- 
Rafter method, hereafter described, was adopted. This method, 
with but slight modifications, has been used to the present 
time. In the course of the whole series of investigations by 
the State Board of Health it is estimated that upwards of 
16,000 samples of water have been submitted to microscopical 
examination—2,000 by the cloth method, 2,000 by the sand 
method, and 12,000 by the Sedgwick-Rafter method. The 
samples cover all parts of the state, and include almost all possi- 
ble sources of water supply—lakes, ponds, artificial reservoirs, 
brooks, rivers, springs, wells, filter galleries, etc. 
Next in importance to the work of the State Board of Health, 
and in some respects surpassing it, is the biological work of the 
Boston Water Supply Department. These investigations have 
involved the examination of more than 20,000 samples of water, 
collected chiefly from the various portions of the city supply. 
The Sedgwick-Rafter method has been used from the start, 
though from time to time several improvements bave been in- 
troduced. 
In the summer of 1893 the Public Water Board of the City 
of Lynn fitted up a biological laboratory in the basement of the 
city hall for the weekly microscopical and bacteriological ex- 
amination of the water supply. The bacteriological work was 
not continued long, but the microscopical examinations are 
still being made. The total number of examinations by the 
Sedgwick-Rafter method now exceeds 2,000. 
The grand total of the number of microscopical examina- 
tions made in these three laboratories added to a large number 
made by F. F. Forbes in his studies of the Brookline water, 
and by others interested in the subject, is not far from 40,000- 
