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their shape, make their outlines less distinct, break up
their arrangement, and disturb them in a variety of other
ways. Because of the possible errors of appearance re-
sulting from these causes, as well as because it must be
determined whether or not the individual is motile, in.
making a careful study of a bacterium it must always be
examined in the living, unstained condition.
The simplest method of making such an examination
would be to take a drop of the liquid, place it upon a
slide, put on a cover, and examine.
While this method is simple, it cannot be recommended,
for if the specimen should need to be kept for a time
much evaporation takes place at the edges of the cover-
glass, and in the course of an hour or two has changed it
too much for further use. The immediate occurrence of
evaporation at the edges also causes currents of liquid to
flow to and fro beneath the cover, carrying the bacteria
with them and making it almost impossible to determine
whether the organisms under examination are motile or
The best way to examine living micro-organisms is in
what is called the hanging drop (Fig. 6). A hollow-
FIG. 6.—The " hanging drop " seen from above and in profile.
ground slide is used, and with the aid of a small camePs-
hair pencil a ring of vaselin is drawn on the slide about,
not in, the concavity at its centre. A drop of the mate-
rial to be examined is placed in the centre of a large
clean cover-glass, and then placed upon the slide so