IV. 
STAINING METHODS. 
THE rapid development of our knowledge with reference to the 
minute microédrganisms under consideration depends very largely 
upon the discovery that they may be stained by various dyes, and es- 
pecially by the aniline colors. Weigert (1876) was the first to employ 
these colors in studying the bacteria, and Koch at once recognized 
the value of the method and made use of it in his researches. 
The basic aniline colors are those employed, and among these the 
most useful are fuchsin, methylene blue, gentian violet, Bismarck 
brown, and vesuvin. 
Staining upon the Cover Glass or Slide.—By a ‘‘ cover-glass 
preparation” we mean that material supposed to contain bacteria 
has been spread out upon a thin glass cover, dried, and stained for 
microscopical examination. A small drop of a liquid culture may, for 
oe 
Fie. 13. 
example, be spread upon a perfectly clean cover glass by means of a 
platinum wire held in a glass handle (Fig. 13). Or we may place a 
drop of water in the centre of the thin glass cover, and by means of 
the same instrument take a little material from a culture made upon 
the surface of a solid medium and distribute it through the drop. 
In this case we must be careful to take very little of the material, as 
the smallest quantity will contain an immense number of bacteria, 
and for a satisfactory view of the individual cells it is necessary that 
they be well separated from each other, in some parts of the prepa- 
ration at least, and not massed together. 
Where the object is to make a cabinet preparation for permanent 
preservation, special care should be taken to distribute the bacteria 
uniformly through the drop of water. The next step consists in eva- 
porating the liquid so that the bacteria may remain attached to the 
surface of the glass cover. This may be done by simple exposure to 
the air or by the application of gentle heat. When the bacteria are 
