74 BACTERIOLOGICAL DIAGNOSIS. 
RECOGNITION OF THE TUBERCLE BACILLUS. 
The tubercle bacillus is about half as long as a red 
blood corpuscle is wide, or rather longer, and is very 
slender. It is straight or slightly curved, and is variable 
both in shape and in size (Plate II., fig. 2). 
We recognise it by means of a staining reaction. 
Tubercle bacilli contain a considerable amount of fat, 
and this prevents them from staining readily with 
ordinary stains. In the process described above we 
used fuchsin, which is a very powerful stain, and added 
a mordant (carbolic acid) which increases its penetra- 
tive properties. Even with this, staining is very slow, 
so that we heated the specimen. 
The fat which prevents the bacilli from staining also 
prevents the stain from being removed by such sub- 
stances as acids and alcohol. In stage 3 of the above 
process we aim at allowing the acid to act until it has 
removed the fuchsin from everything except the tubercle 
bacilli. The methylene blue is a counter-stain, and 
colours all organisms, pus cells (especially their nuclei), 
epithelial cells, and shreds of lung tissue; in fact every- 
thing except the tubercle bacilli. The latter appear as 
slender red rods which often show the irregular staining 
which has been described as occurring in the diphtheria 
bacillus. 
Now “acid-fast” bacilli are very rare, though they 
have been found in unexpected situations of late years. 
Only three such bacilli need be taken into consideration 
in dealing with human pathology.’ These are the 
tubercle bacillus, the leprosy bacillus, and the smegma 
bacillus. The bacillus of leprosy would rarely lead to 
