116 ELEMENTS OF APPLIED MICROSCOPY. 
examination of blood smears for malaria the bacteriolo- 
gist of a Board of Health is expected to make diagnoses 
of diphtheria and tuberculosis, which are of special im- 
portance on account of the necessity for prompt treat- 
ment of the former disease and on account of the difficulty 
sometimes experienced in recognizing the latter. Both 
maladies are due to the presence of bacteria, very minute 
rod-shaped fungi which first gain lodgment in the respira- 
tory or alimentary passages. 
In the case of consumption or pulmonary tuberculosis 
the bacteria are discharged in enormous numbers in the 
sputum and may there be detected without great difficulty. 
When stained with an anilin-dye the tubercle bacilli are 
not decolorized by dilute sulphuric acid as are most bac- 
teria, and this property is made the basis for a simple 
method of differential staining. A thick smear of sputum 
is made on a glass slide and dried over the flame. This 
is stained for two minutes, heating until it steams, with 
Zichl-Neelsen’s carbol fuchsin (1 gram basic fuchsin in 
Io cc. 95% alcohol mixed with 90 cc. of a 5% aqueous 
solution of phenol). ‘The slide is then washed under the 
tap and immersed for two minutes in a solution of 3 parts 
of hydrochloric acid in 100 parts of 95% alcohol. It is 
then washed again and counterstained for half a minute 
in a 1% aqueous solution of methylene blue, washed, 
dried, and mounted. The slender tubercle bacilli should 
be stained bright red with fuchsin, while other bacteria 
and the leucocytes and other cell elements present are 
colored faintly blue. 
In all work with bacteria, the ;'g-inch objective must 
