THE MICROSCOPE AND MICROSCOPIC METHODS 49 



bacilli may be obscured by refraction in the thicker portions of 

 the smear. Tubercle bacilli take a brilliant red color; other_ 

 bacteria and the nuclei of cells are stained blue. 



Of the numerous methods of staining tubercle bacilli only 

 a few others can be mentioned. Aniline- water fuchsin, aniline- 

 water gentian violet, or carbol-fuchsin may be used. The in- 

 tensity of the stain must then be increased by warming the prepa- 

 ration till it steams or boils, then allowing the warm stain to 

 act on the specimens for from three to five minutes; the prepara- 

 tion may also be left in the cold stain over night. Decoloriza- 

 tion may be effected with a 25 per cent solution of sulphuric 

 acid used till the red color disappears, or a 30 per cent solution 

 of nitric acid, which operates very rapidly. If the red color 

 persists after washing in water, dip in the acid again. After 

 either acid the preparation is to be washed in alcohol until the 

 last trace of the stain has been removed. An excellent de- 

 colorizing agent is a 3 per cent solution of hydrochloric acid in 

 alcohol, used for about a minute. The contrast stain may be 

 omitted entirely if it is .desired. A suitable contrast stain after 

 fuchsin staining is a solution of methylene blue; after gentian- 

 violet staining, Bismarck brown. 



Those who have had experience in staining tubercle bacilli 

 soon discover that the bacilli exhibit some differences in their 

 resisting power to strong acids. One encounters occasionally 

 bacilli that are perfectly stained side by side with others that are 

 more or less completely decolorized. These facts show the 

 necessity of practice with any method, and of exercising caution 

 and judgment in making a diagnosis where the number of bacilli 

 happens to be scanty. If tubercle bacilli are not found in the 

 first preparation, other preparations should be made. Some- 

 times a large number of cover-glasses must be examined. 



Various expedients have been devised to concentrate tubercle 

 bacilli when only a small number may be present in a sample of 

 sputum. Recently, antiformin (a preparation of chlorinated 

 sodium hydroxide) has been employed for this purpose. The fol- 



