ESSAYS ON BACTERIOLOGY. 127 



process by wliicli the tubercle bacilli are recognized 

 for diagnostic purposes. Spares usually require the 

 application of the more energetic staining processes, 

 and resist the action of decolorizing agents more than 

 mature bacteria. . The staining of bacteria in thin 

 sections of tissiie is accomplished by the method just 

 described, but requires more time, more care in the 

 use of decolorizing agents, and often contrast colors 

 for the tissue. For the latter purpose we use one of 

 the acid anilin dyes, such as eosin, or a tissue stain like 

 carmine or hematoxylin. Double staining is the 

 process of giving one color to one variety of bacteria 

 and another contrast color to the others or to the sur- 

 rounding field. It is often of aid to the eye in locat- 

 ing and distinguishing a particular variety under the 

 microscope. 



To illustrate these processes we prepare a number 

 of specimens upon thin cover glasses, spreading, dry- 

 ing and heating them so that all are ready for stain- 

 ing. We now test tipon them the dyes prepared as 

 described above, a few drops of staining solution be- 

 ing placed upon each glass. After a few minutes the 

 glasses are washed in water, the clean faces are wiped 

 dry, and they are ready for inspection. The first 

 specimens have been subjected to a cold watery solu- 

 tion of the dye. Examination shows most of the bac- 

 teria to have taken up the color. Several varieties re- 

 main colorless. A repetition of the process, except 

 that the glasses are held in forceps, prepared-side up- 



