Ii6 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 

 Ziehl-Neelsen's carbol fuchsin (i gram basic fuchsin in 

 ID 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 V^-inch objective must 



