MICROSCOPIC METHODS 81 



Examination of Bacteria injthe Hanging Drop. It is often valuable to 

 observe bacteria alive, so as to study them under natural conditions. 

 \Y<- can thus note the method and rate of their multiplication, the 

 pivsi'iice or absence of spore formation, and their motility in fluids and 

 their agglutination with specific serums. For this, special slides and 

 methods are desirable. The usual form is one in which there is ground 

 out on one surface a hollow having a diameter of about half an inch 

 (Fig. 57). According to the purpose for which the hanging drop is to 



FIG. 57 



Hollow slide with cover-glass. 



be studied, sterilization of the slide and cover-glass may or may not 

 be necessary. The hanging block has already been described. The 

 technique of preparing and studying the hanging drop is as follows : The 

 surface of the glass around the hollow in the slide is smeared with a 

 little vaselin or other inert substance. This has for its purpose both 

 the sticking of the cover-glass to the slide and the prevention of 

 evaporation in the drop placed in the little chamber, which is to be 

 formed between the cover-glass when placed over the hollow, and the 

 slide. 



For the purpose of studying the bacteria we place, if they are in 

 fluids, simply a platinum loopful upon the centre of the cover-glass and 

 then invert it by means of a slender pair of forceps over the hollow in the 

 slide, being very careful to have the bacteria over the very centre of the 

 space. If the bacteria, on the contrary, are growing on solid media, or are 

 obtained from thick pus or tissues from organs, they are mixed with a 

 suitable amount of bouillon or sterile physiological salt solution either 

 before or after being placed upon the cover-glass. If we wish to observe 

 the bacteria under natural conditions we must keep the tiny drop of fluid 

 at the proper temperature for the best growth of the bacteria. If, 

 however, we simply wish to observe their form and arrangement this is 

 not necessary. 



In the study of living bacteria we often wish to observe their group- 

 ing and motion rather than their individual characters, and so use less 

 magnification than for stained bacteria. In studying unstained bac- 

 teria and tissues we shut off as large a portion of the light with our 

 diaphragm as is compatible with distinct vision, and thus favor con- 

 trasts which appear as lights and shadows, due to the differences in 

 light transmission of the different materials under examination. It is 

 necessary to remember thai they are seen with difficulty, and that we 

 are very apt, unless extremely careful in focusing, to allow the lens 

 to go too far, and so come upon the cover-glass, break it, destroy our 

 preparation, and, if examining parasitic bacteria, infect the lens. This 

 may be avoided by first finding the hanging drop with a low-power lens 

 and thus exactly centre it. The lens of higher magnification is now 



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