34 BACTERIOLOGY 



softened and then drawn out into a capillary of any desired 

 length and caliber. This is melted in the middle, and severed 

 by the flame, giving two pipettes. When a large capacity is 

 desired a bulb may be blown in the tube between the' capillary 

 and the cotton plug. This requires a little practice. The tip 

 of the pipette is finally broken off with aid of a file, sterilized by 

 the flame and the pipette is ready for use. The various steps in 

 the preparation of pipettes are illustrated in the figures (Fig. 2 1) . 

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

 the microscope while suspended in some fluid substance. The 

 platinum loop having been heated to a red heat in the flame and 

 having been allowed to cool, a small portion of the culture or 

 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 



Fig. 22. 



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 

 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 



