180 B A CTERIOL OG Y. 



color, it may be dipped in alcohol, again washed in 

 water, and stained with some contrast-color. If, for 

 example, the tubercle bacilli have been stained with 

 fuchsin, methylene-blue forms a good contrast-stain. 

 In making the contrast-stain the steps in the process 

 are exactly those followed in the ordinary staining 

 of cover-slip preparations in general : the slip contain- 

 ing the stained tubercle bacilli is carefully rinsed in 

 water, and a few drops of the methylene-blue solution 

 placed upon it and allowed to remain for thirty or 

 forty seconds, when it is again rinsed in water and 

 examined microscopically. For this purpose of observ- 

 ing the difference in behavior of the tubercle bacilli 

 and the other organisms present in the preparation 

 toward this method of staining, it is well to exam- 

 ine the preparation microscopically before the con- 

 trast-stain is made; then give it the contrast-color, 

 and again examine. It will be seen that before the 

 contrast-color has been given to the preparation the 

 tubercle bacilli are the only stained objects to be 

 made out, and the preparation appears devoid of 

 other organisms ; but upon examining it after it has 

 received the contrast-color a great many other or- 

 ganisms will appear; these take on the second color 

 employed, while the tubercle bacilli retain their orig- 

 inal color. Before decolorization all organisms in the 

 preparation were of the same color, but during the appli- 

 cation of the decolorizing solution all except the tubercle 

 bacilli gave up their color. This micro-chemical charac- 

 teristic, together with reactions to be described, serves 

 to differentiate the tubercle bacillus from other organ- 

 isms with which it might be confounded. A number 

 of different methods have been suggested for the stain- 



