AGf~ICULTURAL RESEARCH, PUSA.

STAINING METHODS
As bacteria consist of clear protoplasmic matter,
differing but slightly in refractive index from the
medium in which they are grmvn, it is difficult, except
by special methods of illumination, to sce them in the
uIlstained condition.
Staining, therefore, is of importanee, not only for
the recognition of bacteria, but also in virtue of the
fact that by special methods certain organisms, sllch
as tubercle and diphtheria bacilli, may bc differentiated
microscopically from others.
Bacterial protoplasm reacts to stains in a manner
similar to the nuclear material of tissue cells, and
therefore the various basic aniline dyes arc the
comrnonest stains employed. The action of these
stains may be intensified by the use of mordants such
as phcnol 01' weak alkalies, by the application ot' heat,
or by prolonging the time of staining. Some organisms
have a greater aflinity for dyes than others, so
that when stained and then treated with a decolorising
agent they still retain the stain while others lose it.
By such means it is often possible to differentiate
varieties of bacteria in n mixture.
METHODS OF MAKING FILl\l OR SMEAR
PREPARATIONS
Before taking up the various staining processes,
det.ails of the methods employed in making film.s
must be considered.
Film preparations are made eit.her on cover-slips or
on the ordinary 3 x 1 in. glass slides. It is essential that
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