248 



THE INFECTIOUS DISEASES. 



the influence of the acid the specimen on the cover glass loses its red color and becomes 

 gray or colorless. It is then thoroughly rinsed in three or four successive portions of 

 alcohol, and finally in water. By this manipulation the red color may be to a slight 

 extent restored. 



Care should be taken not to expose the specimen too long to the action of the acid, 

 because then the bacilli may be also partially or completely decolorized. A little ex- 

 perience will enable the experimenter to judge of the proper time for the action of the 



FIG. 122. 



FIG. 122. CULTURE OF TUBERCLE BACILLUS ON GLYCERIN AGAR. From tuberculosis in man. 

 FIG. 123. CULTURE OF TUBERCLE BACILLUS ON GLYCERIN AGAK. From tuberculosis in the bird. 



acid. The specimens may be studied in -water with the use of an oil immersion lens 

 and the Abbe condenser, or thty may be dried in the air and mounted in balsam before 

 examination. 



Inasmuch as not infrequently some other bacteria besides the tubercle bacilli retain 

 a slight red color, it is well, after the specimen is rinsed in water, to float the cover 

 glass for a few minutes in a dilute aqueous solution of methylene blue, which will re- 

 place the red color in all of the bacteria except the tubercle bacilli and which might be 

 mistaken for it, forming a marked color contrast between them. The contrast stain 

 should not be intense. 



IN SECTIONS. Thin sections of tuberculous tissue which has been hardened in alco- 

 hol are stained in the same way, except that instead of drying and fixation by heat the 

 sections should be fixed to the cover glass by means of the albumin fixative (see page 

 ol), and then cover glass and section are manipulated together. 



When differentiation is complete the section is cleared in oil of origanum and 

 mounted in balsam. 



