Juday—Limnological Apparatus. 
587 
organisms present, but also of the relative abundance of the 
different forms. For those organisms which are so small 
that a compound microscope is necessary to count them, 
the regular Sedgwick-Rafter 1 counting cell and ocular mi¬ 
crometer are used. In counting the nannoplankton organ¬ 
isms the sample of water is thoroughly shaken in order to 
get a uniform distribution of the organisms. One cubic 
centimeter is then removed and placed in the cell. A num¬ 
ber of squares, from 20 to 40, are counted in order to ascertain 
the number of the more abundant forms. For the less 
abundant forms, samples of water are placed in sedimenta¬ 
tion tubes having a capacity of 15 cc. and centrifuged about 
6 minutes at a speed of 3,600 revolutions per minute. The 
material collected in the bottom of the tube together with 
about 1 cc. of water, is removed with a pipette and trans¬ 
ferred to a cell for counting. Frequently a second centri¬ 
fuging is necessary in order to bring down all of the organ¬ 
isms. The mean of duplicate or triplicate determinations 
made in this manner will give a good idea of the abundance 
of the various forms in the sample of water. 
The catch of net plankton is diluted to a definite volume, 
usually 10 cc. or 20 cc. depending upon the abundance of 
the material. Then it is thorougly shaken and 1 cc. is trans¬ 
ferred to the Sedgwick-Rafter cell for the enumeration of 
the smaller organisms. 
For the larger organisms, such as Crustacea and rotifers, 
the material is again thoroughly shaken and 2 cc. or 4 cc., 
depending on the degree of dilution, are removed with a pis¬ 
ton pipette for counting under the low power of a binocular 
microscope. The cell used in counting this material con¬ 
sists of a brass frame which is cemented to a regular glass 
slide with sodium or potassium silicate. This cement serves 
admirably for alcoholic material but it is so soluble in water 
that it can not be used for fresh or formalin material. The 
brass frame is 75 mm. long, 25 mm. wide, and 2 mm. thick, 
with an opening in the middle which is 63 mm. long and 
nearly 8 mm. wide. The width of this opening is such that 
it just covers the field of the binocular microscope, so that a 
form may be counted simply by passing the cell through the 
field of the binocular once. 
1 See Whipple, Microscopy of Drinking Water, p. 34, 1914. 
