36 BACTERIOLOGY 



instead of a drop of fluid. Bacteria are distributed on the sur- 

 face of the agar, which is then apphed to a cover-glass, and 

 mounted like a hanging-drop. The bacteria are thus kept in a 

 layer close to the glass, where growth may be studied. 



The Microscopic Preparation for Study by Dark-field lUumi- 

 nation.^ — The central portion of a clean glass slide is encircled 

 with a ring of vaseline, and a drop of the fluid to be examined 

 is deposited on the clean surface in the center of the ring by means 

 of a capillary tube. It is then covered with a clean large cover- 

 glass so that the fluid spreads out in a moderately thin layer 

 beneath the cover-glass and is confined on all sides by the vaseline, 

 thus preventing evaporation and resulting currents in the 

 preparation. 



Best results with the dark-field microscope are obtained only 

 in a dark or dimly lighted room. An electric arc or a powerful 

 . gas-light or, better still, a powerful daylight-glass nitrogen-bulb 

 incandescent electric light, may be employed as the source of light, 

 and it is well to put a flask of water between the light and the 

 microscope to eliminate the heat-rays. The substage condenser 

 of the microscope is replaced with the special dark-field condenser 

 and this is carefully centered. A large drop of immersion oil is 

 placed on the upper surface of the condenser. The slide is 

 carefully placed upon the stage so that the oil fills in completely the 

 space beween the condenser and slide and remains free from air- 

 bubbles. The preparation is then ready for examination. Objec- 

 tives of numerical aperture wider than i.o cannot be successfully 

 used with the ordinary dark-ground condensers and therefore it is 

 necessary to stop down the aperture of the oil-immersion objec- 

 tive before using it. A special funnel stop is furnished for this 

 purpose. When this has been attached the preparation may be 

 studied with the oil-immersion objective in the usual way. Skill 

 in this method of studying unstained microbes is quickly acquired, 

 offering, as a rule, less difficulty than the method of central 

 illumination which is employed for the hanging-drop and hanging- 

 block. 



