PROTOZOAN FAUNA OF HAY INFUSIONS 217 
course, and such error as existed from this would only be augmen- 
ted by passing the pipet more than once through this region. 
Various methods were tried to avoid this error entirely. For 
example, when the study of a sample suggested that possibly some 
of the organisms observed might have entered from the surface 
fluid, another sample was taken with a pipet in the tip of which a 
cork was inserted. When the pipet in this condition had reached 
the point from which the sample was desired, a wire was inserted 
through the pipet and the cork pushed out. The pipet, of course, 
immediately filled with water up to the level of the surrounding 
infusion and the cork itself rose to the surface. In the great 
majority of cases it was found that samples taken by this latter 
method simply corroborated those taken by the more expeditious 
means, and consequently it is believed that the data secured with 
the method generally used in the work possesses an error which is 
negligible. ; 
After a sample of the infusion had been removed it was imme- 
diately put into a watch glass and stirred, and then 1 ec. was 
taken with a pipet and put into a Sedgwick-Rafter counting cell. 
As is well known, this consists of a glass slide upon which is ce- 
mented a metal rectangle. The dimensions of the space enclosed 
by the rectangle is 50 x 20 mm., and, as the metal is 1 mm. thick, 
when the rectangle supports a large cover glass it forms a cell 
which has a capacity of exactly 1 cc. The sample to be examined, 
then, was spread out on the slide to a depth of 1 mm., and pre- 
sented to view a total of 1000 cubic mm. The contents of this 
cell was then at once examined under a microscope which was 
provided with an ocular micrometer so ruled that, with lenses and 
tube length properly adjusted, a square of the micrometer just 
covered 1 sq. mm. of the field, and by focussing through the depth 
of the liquid enclosed by the square, a volume of the sample equal 
to 1 cu. mm. was under observation. By counting the organisms 
which were included, during a unit of time, in the 1 cu. mm. under 
observation, and multiplying this by 1000, the number of organ- 
isms in the cell could be ascertained. Usually ten such counts, 
* For a detailed description of the apparatus, cf. Whipple: The microscopy of 
drinking water, 2d ed. 1910. 
