STAINING THE TUBERCLE BACILLUS. 143 



immersed in a 30 per cent, solution of nitric acid in 

 water and allowed to remain there until all the color 

 has disappeared. In some cases this takes longer than 

 in others. One can always determine if decolorization 

 is complete by washing off the acid in a stream of water. 

 If the preparation is still quite colored it should be again 

 immersed in the acid ; if of only a very faint color it 

 may be dipped in alcohol, again washed off in water, and 

 may now be 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 containing the 

 stained tubercle bacilli is rinsed off carefully in water, 

 and a few drops of the me thy leu e- blue solution are 

 placed upon it and allowed to remain for thirty to forty 

 seconds, when it is again rinsed in water and examined 

 microscopically. For the purpose of observing the 

 difference between the behavior of the tubercle bacilli 

 and the other organisms present in the preparation 

 toward this method of staining, it is well to examine 

 the preparation microscopically before the contrast stain 

 is made, then remove it, give it the contrast color, and 

 examine it again. It will be seen that before the con- 

 trast color has been given to the preparation the 

 tubercle bacilli will be the only stained objects to be 

 made out, and the preparation will appear devoid of 

 other organisms, but upon examining it after it has 

 received the contrast color a great many other or- 

 ganisms will now appear ; these will take on the second 

 color employed, while the tubercle bacilli will retain 

 their original color. Before decolorization all organisms 



