34 BACTERIOLOGY 



other material may be removed with it and deposited in the center 

 of an ordinary cover-glass. The needle should again be sterilized 

 in the flame. When cultures on solid media are to be examined, 

 a small particle may be mixed with a drop of sterilized water or 

 bouillon. The cover-glass should have been carefully cleaned and 

 sterilized over the flame. The cover-glass with the small drop 

 of fluid material held in sterilized forceps is now to be inverted 

 over a sterilized glass slide, which has a concavity ground in the 

 middle of it. Around the concavity, the slide should be smeared 

 with vaseline. In this manner a small air-tight chamber is made. 

 This slide and cover-glass is next put upon the stage of the micro- 

 scope. A good dry lens, if of sufficiently high power, is more 

 convenient for examining the hanging-drop than an oil-immer- 

 sion. If the latter be used, having placed a drop of cedar-oil on 

 the center of the cover-glass, and a good light having been 



FIG. 22. 



secured, the oil-immersion objective should be brought down 

 upon this drop of oil. The beginner often experiences difficulty 

 in focusing upon a hanging-drop. It is necessary to shut off 

 most of the light by means of the iris diaphragm, for as has 

 already been pointed out (page 28), colorless objects may be clearly 

 seen only when illuminated either by a narrow central beam or 

 by oblique illumination (dark-field). Often it is well to secure 

 the focus roughly upon the extreme outer edge of the chamber, 

 or to find the edge of the drop of fluid with the low power and 

 then focus upon this edge with the oil-immersion objective. 

 Above all things guard against breaking the cover-glass by forcing 

 the objective down upon it. The motility of certain bacteria is one 

 of the most striking phenomena to be observed in the hanging- 

 drop. It is not to be confused with the so-called "Brownian 

 movement' ' which is exhibited by fine particles suspended in a 



