94 METHODS USED IN THE STUDY OF BACTERIA 



nearest the observer's eye is known as the eye-piece or ocular. 

 Two of the most serious defects of lenses are what are known 

 as spherical and chromatic aberration. An oil-immersion 

 lens and an Abbe substage condenser are essential parts of 

 a bacteriological microscope. 



The Examination of Bacteria. Bacteria may be examined 

 under the microscope in a living condition or stained. They are 

 examined in a living condition in what are known as hanging 

 drop preparations. These are prepared by taking a drop of the 

 fluid culture, or an emulsion of the bacteria, and putting it on a 

 cover-glass. This cover-glass is then inverted over the cavity 

 in a hollow ground glass slide. The edge of this cavity is oiled 

 so as to prevent evaporation. The bacteria in the hanging 

 drop are thus furnished with opportunity to move about, 

 or to remain in a natural condition. This preparation en- 

 ables the observer to determine whether or not there is vital 

 movement, and also to detect other subtle peculiarities, so that 

 this preparation is one. of great importance. The beginner 

 frequently finds difficulty in making it because of the fact 

 that the bacteria may be some distance from the top of the 

 cover-glass and thus the working distance of the lens is very 

 slight, also because of the difficulty in regulating the amount 

 of light, since these bacteria are so nearly transparent that a 

 little too much light renders them invisible. 



Bacteria are also examined in the stained condition by what is 

 known as a cover-glass preparation. Staining methods were not 

 introduced until the latter part of the last century, but since their 



