EXAMINATION OF BACTERIA WITH THE MICROSCOPE. 23 



The Hanging-drop. Living bacteria may- be studied with 

 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 physiological salt solution or bouillon which has first 

 been placed in the middle of the cover-glass. The cover-glass 

 should have been carefully cleaned and sterilized over the 



FIG. 6. Diagram of the hanging-drop. 



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 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 micro- 

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

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

 mersion. 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 



