114 PRACTICAL BACTERIOLOGY. 



Upon this variation of the powers of the bacteria 

 of absorbing and retaining stains, the method of 

 double or contrast staining is based. The following 

 experiment serves to show how this method may be 

 used with success. A minute quantity of a colony 

 of some fluorescent bacteriiimj which is not readily 

 stained, is mixed in a drop of water upon a cover- 

 glass with a small quantity of another colony of a 

 bacterium which is readily stained, and which differs 

 morphologically from the first. The preparation, 

 after it has been dried and fixed, is then left for five 

 minutes in aniline water solution of fuchsine. This is 

 best done by placing a few drops of the solution upon 

 the cover-glass, and leaving them there for five 

 minutes. The cover-glass is then washed with water, 

 and left in dilute alcohol until the more easily 

 stained bacteria have lost their coloration. The 

 cover-glass is then again washed, after which it is 

 treated with an aqueous solution of methylene blue 

 for about ten minutes. 



When now the cover-glass is examined with the 

 microscope, it will be seen that while the fluorescent 

 bacteria have retained the red colour of the fuchsine, 

 even though only faintly (the intensity varies with the 

 species), and have not absorbed any of the methylene 

 blue, being unaffected by aqueous solutions, the other 

 kind have lost their red colour completely in the 

 alcohol, and are of an intense blue' colour. Thus, red 



