I.] MICROSCOPIC EXAMINATION. n 



A similar result is often obtained by mixing the two dyes, 

 and then using them like a single dye ; hereby occasionally 

 the bacteria are found to take one colour, while the tissue- 

 elements take the contrast dye. 



2. One of the most useful methods for staining bacteria 

 in sections of hardened tissues is Gram's method. Sections 

 are kept for ten minutes in absolute alcohol, are then placed 

 in any of the above mixtures of aniline oil and dye (fuchsin, 

 magenta, Humboldt's red or gentian-violet, methyl-blue or 

 methyl-violet), and kept here for from two to five minutes or 

 more ; they are then washed in alcohol for from one to three 

 minutes, and are then transferred into the following solution : 

 one part iodine, two parts of iodide of potash, 300 parts of 

 distilled water : they are kept here till their colour completely 

 changes (as a rule into dark purple), they are then transferred 

 into alcohol till all colour has apparently gone. If success- 

 ful, such sections when examined under the microscope, 

 show only the bacteria stained, while the tissue-elements are 

 quite colourless. To bring out these latter more strikingly 

 the sections are stained in a contrast dye, vesuvin or 

 Bismarck-brown, if red, violet, or blue has been used as the 

 first dye. 



3. Ehrlich's method, used specially for demonstrating 

 tubercle-bacilli and leprosy-bacilli. The specimens, after 

 having been well stained (from a few minutes to several 

 hours) with fuchsin or magenta aniline oil dye (see above), 

 are transferred into a 30 per cent, mixture of nitric acid ; 

 according to Friedlander a mixture of three parts of nitric acid 

 in 100 parts of alcohol is equally good. A 10 per cent, 

 watery solution of nitric acid is quite strong enough. All 

 bacteria except the tubercle-bacilli and leprosy-bacilli lose 

 the dye by this treatment. The preparations are then 

 stained for contrast in vesuvin or Bismarck -brown. 



