Juty 16, 1897. ] 
square of such a size as to cover one sq. 
mm. on the stage, 7. ¢., one thousandth of 
the entire area of the cell. The organisms 
observed within the limits of the ruled 
square are then counted and the cell moved 
until another portion comes into view, 
when another count is made. Thus 10 or 
20 squares are counted and the number of 
organisms present in the sample calculated. 
This process has many things to be said 
in its favor, and it is undoubtedly the best 
all-around method for the study of the 
plankton. The apparatus required is simple, 
inexpensive and not liable to get out of 
order. The process is neither long nor 
difficult, and if care and cleanliness are ob- 
served in the manipulation very accurate 
results may be obtained. Ordinarily the 
quantity of water operated upon is small, 
but there is no reason why large filters may 
not be used. The writer has frequently 
used a funnel having a neck one inch in 
diameter, filtering from 1,000 to 10,000 e.c. 
This, when used with an aspirator to hasten 
the filtration, has given excellent satisfac- 
tion. The chief objection to the Sedgwick- 
Rafter method is that delicate organisms 
are liable to be crushed upon the sand, and 
this danger is naturally somewhat greater 
when this aspirator is used. It is probably 
no greater, however, than in Reighard’s 
net. : 
Recently a new apparatus has been de- 
vised for the study of the microscopical or- 
ganisms, known as the planktonikrit. This 
isa modification of the centrifugal machine 
and depends upon the fact that the specific 
gravity of the organisms is different from 
that of water. It has the advantage of 
avoiding, to a certain degree, the crushing 
of the delicate infusoria, but it is somewhat 
inaccurate in the case of some of the lighter 
organisms; furthermore, it operates upon 
very small quantities of water. 
In a complete study of the microscopical 
organisms, such as might be undertaken on 
SCIENCE. 
87 
cur great lakes, for example, it would be 
advisable to use all three methods, adopting 
the Sedgwick-Rafter method for general 
quantitative work, but using the net and 
centrifugal apparatus for determining the 
rare and delicate organisms. 
As there are many lovers of the micro- 
scope who are interested in studying aquatic 
life, and as there are many others con- 
nected with water-works to whom the study 
of algeze and infusoria would be of much 
value, the writer has tried to reduce the 
Sedewick-Rafter method to its simplest 
possible elements in order that it may be 
more generally used. Furthermore, it is 
often necessary for the sanitary biologist to 
be provided with a portable outfit for work 
in the field. There are many fragile organ- 
isms which will not bear transportation to 
the laboratory. Uroglena, for example, a 
very important and troublesome organism 
found in water supplies, goes to pieces com- 
pletely when kept for a short time in a 
stoppered bottle. It is, therefore, necessary 
to make the examination of water immedi- 
ately after the collection of the sample. 
The chief modification of the method for . 
field work consists in the use of a cylindri- 
eal glass funnel [Fig. 1] similar to the one 
designed by Mr. D. D. Jackson for the 
Massachusetts State Board of Health, but 
differing from it in having a capacity of 
250 instead of 500 ¢c.c., and in having 
graduations marked upon the sides. This 
funnel may be conveniently carried and its 
graduation renders the use of a second 
measuring glass unnecessary. When in 
use it may be supported on a wire frame, 
which any ingenious person can make. In 
place of the test-tube it has been found 
convenient to use tube vials [Fig. 2] hav- 
ing square ends. These require no racks 
and are not easily tipped over. The pi- 
pette for washing the sand might be dis- 
pensed with if one of the tube vials was 
graduated, but as much depends upon ac- 
