30 BACTERIOLOGY 



and examined with the oil-immersion lens. Or it may 

 be examined by the hanging-drop method. In this 

 method a slide is used in which a small depression is 

 hollowed out. The drop of culture or pus to be exam- 

 ined is placed upon the cover-glass, which is then inverted 

 over the depression, and the drop allowed to hang free 

 in the space between the cover-glass and slide. By this 

 means the movement of motile bacteria and agglutina- 

 tion, as in the Widal test, may be observed. By sealing 

 the edges of the cover-glass, evaporation is prevented, 

 and the specimen may be studied for days, and such 

 processes as spore formation and fission observed. 



If it be desired to stain the specimen, it must be pre- 

 pared in the form of a smear. A small amount of the 

 material is taken on a platinum loop (if liquid) or needle 

 (if solid) and smeared over the surface of a cover-glass. 

 It is then fixed by being passed through an open flame, 

 thus coagulating the albumin of the material and fasten- 

 ing it to the cover-glass. The proper staining fluid is 

 then flowed over the smear and allowed to remain the 

 required length of tune, with or without the application 

 of heat, according to the stain used and the specimen 

 examined. When stained, it is placed upon a slide with 

 a drop of water interposed, or if it be desired to mount the 

 specimen permanently, a drop of Canada balsam is 

 used instead of the water between the cover-glass 

 and slide, and the specimen examined in the usual 

 manner. 



