EXAMINATION AND STAINING OF BACTERIA 41 



ground. The method may be employed for the examination of 

 bacteria in general ; it is especially useful for the demonstration 

 of spirochetes in secretions. 



Double Microscopes have been constructed by means of which 

 a comparative study may be made of two objects at the same time. 



Microscopic Examinations. Bacteria may be studied micro- 

 scopically, (1) living and unstained in fluids, (2) in stained film 

 preparations, (3) in stained sections of tissue. For such studies 

 perfectly clean slides and coverslips are necessary. 



Hanging Drop Preparation. In order to note motility or 

 watch the method and rate of cell division it is necessary to study 

 bacteria while they are alive. This can be accomplished by 



means of the so-called hanging 



drop prepared as follows : A spe- 

 cial slide with a circular hollow 

 on one surface is employed and 

 around the edge of the concavity 

 a fine film of cedar oil or vaseline 



is smeared. This latter serves the FIG. 13. Hanging Drop Prepara- 

 purpose of attaching the coverslip 



to the slide and so preventing evaporation. A drop of fluid in 

 which the bacteria are growing is then transferred to the center 

 of a coverslip by means of a platinum loop previously sterilized 

 by flaming. If the bacteria are to be removed from solid media 

 or are obtained from thick pus they are mixed with a suitable 

 quantity of sterile broth or physiological salt solution (0.9 per 

 cent NaCl in distilled water) and a drop of the suspension placed 

 on the coverslip, or the bacteria may be emulsified in a drop of 

 salt solution directly on the coverslip. The coverslip is then 

 inverted over the slide, gently pressed, and sealed by means of the 

 vaseline (Fig. 13). 



Unstained organisms are difficult to see through the microscope, 

 therefore great care is necessary in focusing. The diaphragm 

 should be partially closed in order to take advantage of the lights 

 and shadows caused by the difference in light transmission in the 

 objects under examination. Since the edge of the hanging drop 



