EXAMINATION OF BACTERIA WITH THE MICROSCOPE. 23 



the microscope while suspended in some fluid substance. This 

 is accomplished by means of a hanging-drop. In order to 

 prepare a hanging-drop for examination a clean cover-glass is 

 held in the forceps and a small drop of the fluid to be examined 

 is spread thinly over the center of it by means of a platinum 

 needle which has just previously been heated in a flame and 

 allowed to cool. 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 which has first been placed in the middle of the cover- 

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

 sterihzed over the flame. The cover-glass with the thin 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 



Fig. 6. — Diagram of the Hanging-drop. 



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

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

 made. This preparation, may be put upon the stage of the 

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

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

 immersion. 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 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 well to shut 

 off most of the light by means of the iris diaphragm. 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 to focus upon this edge with the oil-imi- 



