METHODS OF OBSERVING BACTERIA.         89

that the drop hangs in, but does not touch, the concavity.
The micro-organisms are now hermetically sealed in an
air-chamber, and appear under almost the same con-
ditions as in the cul-
ture. Such a speci-
men may be kept
from day to day and
examined, the bac-
teria continuing; to
live until the oxygen
or nutriment is ex-
hausted. By means
of a special appara-
tus (Fig. 7), in which
the microscope is
stood, the growing
bacteria may be
watched at any tem-
perature, and very
exact observations
made.
The hanging drop
should always be ex-
amined at the edge,
as the centre is too
thick.
In such a specimen
it  is   possible   to  de-        fjg. 7.—Apparatus for keeping objects under
termilie    the    shape,    microscopic examination at constant  tempera-
size, grouping, divis-   tures-
ion,   speculation,  and  motility of the  organism   under
observation.
Care should be exercised to use a rather small drop,
especially for the detection of motility, as a large one
vibrates very readily and masks the motility of the
sluggish forms.
When the bacteria to be observed are in solid or semi-
solid culture, a small quantity of the culture should be