210 BACTERIOLOGY. 
the hanging drop is to be studied, sterilization of the 
slide and cover-glass may or may not be necessary. 
The technique of preparing and studying the hanging 
drop is as follows : The surface of the glass around the 
hollow in the slide is smeared with a little vaseline or 
other inert substance. This has for its purpose both 
the sticking of the cover-glass to the slide and the pre- 
vention of evaporation in the drop placed in the little 
chamber, which is to be formed between the cover- 
glass, when placed over the hollow, and the slide. 
For the purpose of studying the bacteria we place, if 
they are in fluids, simply a platinum loopful upon the 
centre of the cover-glass and then invert it by means of 
a slender pair of forceps over the hollow in the slide, 
being very careful to have the bacteria over the very 
centre of the space. If the bacteria, on the contrary, 
are growing on solid media, or are obtained from thick 
pus or tissues from organs, they are mixed with a suit- 
able amount of bouillon or sterile physiological salt 
solution either before or after being placed upon the 
cover-glass. If we wish to observe the bacteria under 
natural conditions we must keep the tiny drop of fluid 
at the proper temperature for the best growth of the 
bacteria. If, however, we simply wish to observe their 
form and arrangement this is not necessary. In the 
study of living bacteria we often wish to observe their 
grouping and motion rather than their individual char- 
acters, and so use less magnification than for stained 
bacteria. In studying unstained bacteria and tissues 
we shut off as large a portion of the light with our 
diaphragm as is compatible with distinct vision, and 
thus favor contrasts which appear as lights and shadows, 
due to the differences in light transmission of the dif- 
