MICROSCOPICAL EXAMINATION. 199 
media, however, on account of the abundance of bacteria 
in the material, a little of the growth is diluted by add- 
ing it to a tiny drop of clean water which has been 
previously placed on the cover-glass. The amount of 
dilution is learned after a few trials. It is best to add 
to the drop just enough to make a perceptible cloudi- 
ness. The mixture is then smeared over the cover- 
glass. From whatever source derived, the film is al- 
lowed to dry at the usual air temperature, and then, in 
order to fix the film with its contained bacteria to the 
glass, the latter is passed three times by a rather slow 
movement through the Bunsen or alcohol flame. In- 
stead of this method, the film may be fixed to the glass 
by placing it in absolute alcohol for a few minutes. 
The smear thus prepared is usually stained either 
by the simple addition of a solution of an aniline dye, 
for from one to five minutes, or by one of the more 
complicated special stains described later, such as that 
of Gram or that used for the tubercle bacillus, where 
the ability of the bacteria to retain their stain when 
placed in decolorizing solutions is tested. When the 
stain is to be hastened or made more intense the dye is 
used warm. For ordinary staining the bacteria are 
simply covered completely by the cold staining fluid. 
The cover-glass, with the charged side uppermost, may 
either rest on the table or be held by some modifica- 
tion of Cornet’s forceps. When the solution is to be 
warmed the cover-glass may be floated, smeared side 
down, upon the fluid contained in a porcelain dish rest- 
ing on a wire mat, supported on a stand, or it may be 
held in the Cornet forceps. The fluid in both the dish 
and on the cover-glass should be carefully warmed, so 
as to steam without actually boiling. The cover-glass 
