36 ROCHESTER ACADEMY OF SCIENCE, [March 10, 
actual enumeration of all the organisms present in a given quantity of 
water. The first point is attained by filtration through a short column 
of fine sand in the lower end of the stem of a small funnel, the sand 
being supported upon some material which will allow the water to pass 
freely and still retain the sand in position. After placing the sand, a 
measured quantity of water is poured into the funnel and allowed to 
filter through. The sand retains nearly all the organisms which were 
originally distributed through the water. The enumeration is secured, 
according to Prof. Sedgwick, by removing the supporting plug and 
washing the sand and contained organisms into a cell, 50 x 20 millime- 
tres in area, and about 2 to 2% millimetres in depth. The glass bottom 
of this cell is ruled into square millimetres and by passing a number of 
these squares through the field of a microscope their contents are 
counted and from the counts so made the whole number present in the 
cell is obtained. This method while far in advance of that of McDonald 
ert sively 
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FIG. o.— FUNNEL WITH PLUG anD SAND IN PLAcg. 
is still somewhat unsatisfactory in this, that the sand and organisms are 
both allowed to pass into the cell together, and inasmuch as the finest 
grains of sand*are much larger than many of the organisms, it follows 
that the enumeration, however carefully made, is only a rough approxi- 
mation to the number actually present, and usually falls short of the 
number actually present. 
