132 BACTERIOLOGY. 



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 con- 

 trast color, and examine it again. It will be seen 

 that before the contrast 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 ex- 

 amining it after it has received the contrast color, a 

 great many other organisms will now appear; these 

 will take on the second color employed, while the 

 tubercle bacilli will retain their original color. Before 

 decolorizatiou all organisms in the preparation were of 

 the same color, but during the application of the decolor- 

 izing solution all except the tubercle bacilli gave up their 

 color. This characteristic, as said, serves to differentiate 

 the tubercle bacillus from all other organisms, except 

 the bacillus of leprosy, which stains in the same way as 

 does the bacillus of tuberculosis. A number of different 

 methods have been suggested for the staining of tuber- 

 cle bacilli, but the original method as employed by 

 Koch is so satisfactory in its results that it is not 

 advisable to substitute others for it. The above differs 

 from the original Koch-Ehrlich method for the staining 

 of tubercle bacilli in sputum only in the occasional em- 

 ployment of Ziehl's carbolic-fuchsin solution and the 

 method of heating the preparation with the staining 

 fluid upon it. 



As Nuttall has pointed out, however, the strong acid 

 decolorizer used in this method can be with advantage 

 replaced by much more dilute solutions, as a certain 

 number of the bacilli are entirely decolorized by the 

 too energetic action of the strong acids. He recom- 



