BACILLUS OF TUBERCULOSIS. 309 
Always make two cover-glass preparations from each 
specimen. Report no result as negative until at least 
two preparations have been subjected to a thorough 
search with a 1/12 oil-immersion or 2 mm. apochromatic 
lens by means of a mechanical stage. From a very 
large experience in the examination of sputum for 
tubercle bacilli, the New York Health Department 
bacteriologists have concluded that the examination 
of two preparations of each specimen in the careful 
manner described above is usually sufficient to demon- 
strate the presence of the bacilli when they are pres- 
ent in the sputa, and they are usually found to be 
present to this extent in fairly well-developed cases 
of pulmonary tuberculosis, and in many cases which 
are in the incipient stage. There are, however, un- 
doubted cases of incipient pulmonary tuberculosis which 
require the examination of many preparations before 
the tubercle bacillus can be found; and that cases also 
occur in which the sputum for a time does not contain the 
bacilli, which were, nevertheless, present at an earlier 
period, and which again later appear. Therefore, if cases 
occur which may be still regarded as possibly tubercu- 
losis, further examinations of the sputum should be 
made. It should also be constantly borne in mind 
that the demonstration of the presence of tubercle 
bacilli in the sputum proves about as conclusively as 
anything can the existence of some degree of tubercu- 
losis; but that the absence of tubercle bacilli or the 
failure to find them microscopically does not positively 
exclude the existence of the disease. Here injections 
of tuberculin can be made use of. 
