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 Léffler’s alkaline methylene-blue— 
Concentrated alcoholic solution of methyl blue 30c.c. 
Caustic potash (1: 10,000 solution ) F . 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 isa 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, 
Ziehl’s 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 
