BACILLUS OF TUBERCULOSIS. 303 



alcohol for from a few seconds up to one minute, re- 

 moving at the time when all color is just about gone 

 from the cover-glass smear. Wash thoroughly with 

 water and make a contrast stain by applying a cold 

 solution of Loffler's alkaline methylene-blue 



Concentrated alcoholic solution of methyl blue 30 c. c. 

 Caustic potash (1:1 0,000 solution) . .100" 



for from fifteen to thirty seconds. Wash with water; 

 press between folds of filter-paper; dry in the air; 

 mount and examine. 



The tubercle bacilli are distinguished by the fact that 

 they retain the red color imparted to them in the 

 fuchsin solution, while the other bacteria present, 

 having been decolorized in the acid solution, take the 

 contrast stain and appear blue. (See plate II., Figs. 

 1 and 2.) 



Various methods have been suggested for the staining 

 of tubercle bacilli, but the original method as employed 

 by Koch, or some slight modification of it, is so satis- 

 factory in its results that it seems unnecessary to substi- 

 tute others for it. The above is a slight modification of 

 the Koch-Ehrlich method, differing from it chiefly in 

 the use of a weak for a strong acid decolorizer. It 

 has been found that the strong acid solution originally 

 employed (5 per cent, sulphuric acid solution in alcohol) 

 often decolorizes some of the bacilli entirely by its too 

 energetic action, and that a weaker decolorizer, such 

 as the above, gives more uniform results. 



Instead of the Koch-Ehrlich aniline-water solution, 

 ZiehVs carbol-fuchsin solution may be used, and is by 

 many preferred (see page 198). Instead of floating 

 the cover-glass smears on the staining fluid they can be 



