56 EXAMINATION AND STAINING OF BACTERIA. 



stoppered bottle because of the volatility of the alcohol. If 

 it becomes too concentrated by evaporation and thus stains 

 too deeply, or forms a precipitate on the blood-smear, the 

 addition of a suitable quantity of rnethylic alcohol will 

 quickly correct such faults. It does not undergo any spon- 

 taneous change other than of concentration by evaporation, 

 according to my personal experience. A most important 

 fault encountered in the working of some samples of this fluid 

 is that it fails to stain the red blood-corpuscles a yellow or 

 orange color, but stains them a blue color, which cannot 

 be readily removed by washing with water. This fault I 

 have recently discovered to be due to a peculiarity of the 

 eosin employed. It can be eliminated by using a proper 

 " yellowish, water-soluble " eosin. 1 



Application of the Staining Fluid to Blood-films. 1 . Cover 

 the film with a noted quantity of the staining fluid by means 

 of a medicine-dropper. 



2. After one minute add to the staining fluid on the film 

 the same quantity of distilled water by means of a medicine- 

 dropper, and allow the mixture to remain for two or three 

 minutes, according to the intensity of the staining desired. 

 A longer period of staining may produce a precipitate. Eosin- 

 ophilic granules are best brought out by a short period of 

 staining. The quantity of the diluted fluid on the preparation 

 should not be so large that some of it runs off. 



3. Wash the preparation in water for thirty seconds, or until 

 the thinner portions of the film become yellow or pink in color. 



4. Dry and mount in balsam. Films more than a few 

 hours old do not stain as well as fresh ones. 



QUESTIONS. 



What powers of the microscope are necessary for the examination of 

 bacteria ? 



How are bacteria examined alive ? 



How is a hanging-drop preparation made ? 



What is the usual method of staining bacteria ? 



What are the most usual stains used for bacteria? 



What is Loeffler's method? 



Describe the Koch-Ehrlich method. 



1 Such an eosin I have obtained from K. L. Emerson, No. 739 Boylston 

 ,St., Boston. 



