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 



