42 EXAMINATION AND STAINING OF BACTERIA. 



of the bacteria may be very well seen and carefully in- 

 vestigated. 



The Hanging-drop Preparation. 



When it is necessary to keep them under observation for 

 some time, however, or when it is desired to study their 

 development, multiplication, or sporulation, what is known as 

 the hanging-drop preparation or culture is resorted to. 



This consists in placing a small drop of the liquid contain- 

 ing the bacteria on a thin cover-glass and placing upon this 

 cover-glass a slide with a concavity in its centre, known as 

 the hanging-drop slide, after having carefully lubricated the 

 edges of the surface around the concavity with vaselin, so 

 that the slide will adhere to the cover-glass when it is pressed 

 down on it. In this way a hermetically sealed, transparent, 

 moist chamber is obtained, which may be kept under observa- 

 tion on the stage of a microscope almost indefinitely. 



Longitudinal section of hollow ground glass slide for observing bacteria in 

 hanging drops. (Abbott.) 



2. The foregoing are very simple and useful methods for 

 rapid examinations of bacteria, and have many applications, 

 but they are far from satisfactory in all cases, as they fail to 

 bring out full details of bacterial structure. For this purpose 

 recourse must be had to the staining methods introduced by 

 Koch, and perfected by Weigert, Loeffler., and many others. 

 In this method bacteria are examined dead. 



THE STAINING OF BACTERIA. 



I. The General Mode of Procedure, 



One or several droplets of the suspected liquid are spread 

 thinly and evenly on the surface of a slide or thin cover-glass, 



