IV. 



STAINING METHODS. 



THE rapid development of our knowledge with reference to the 

 minute microorganisms 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 Crlass 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 



FIG 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 

 unif ormly 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 



