210 BACTERIOLOGY. 



the hanging drop is to be studied, sterilization of the 

 slide and cover-glass may or may not be necessary. 

 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 vaseline or 

 other inert substance. This has for its purpose both 

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

 vention 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 upou 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 suit- 

 able 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 

 grouping and motion rather than their individual char- 

 acters, and so use less magnification than for stained 

 bacteria. In studying unstained bacteria and tissues 

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

 diaphragm as is compatible with distinct vision, and 

 thus favor contrasts which appear as lights and shadows, 

 due to the differences in light transmission of the dif- 



