i62 Methods of Observing Micro-organisms 



Such a thin layer is usually easily obtained by the use of a slide 

 and cover-glass, and the careful preparation of a good film. 



The slide and the cover-glass should be thoroughly cleansed and 

 freed from fat and grit and well polished. A comparatively small 

 drop of blood — let us say, for example — is placed upon the center 

 of the slide and immediately covered with the cover-glass. If the 

 drop is not too large and the glasses are clean, the weight of the cover- 

 glass causes the drop to spread, and capillary attraction completes 

 the formation of a very thin film. The quantity of blood used 

 should not be sufficient to reach the edges of the cover-glass, else 

 sometimes the glass is pressed up instead of being drawn down and 

 moves about too freely. If the examination is to take enough time 

 to cause the drop to dry, a match-stick dipped in thin vaselin and 

 drawn about the edge of the cover will prevent it. 



Such a film is usually best examined at or near the center, where 

 the formed elements are not widely separated. 



The living protozoa in preparations of this kind may be examined 

 by ordinary illumination by transmitted light, or with lateral 

 illumination by means of the "dark-field illuminator." The latter 

 serves better for the discovery of the very small transparent organ- 

 isms — spirochseta and treponema — and for the observation of the 

 cilia and flagella. 



STAINING PROTOZOA 



It is through the study of stained protozoa that we arrive at most 

 of our knowledge of their structural details. They can be stained 

 in blood or fluids upon a slide or in sections of tissue. 



As in the case of the bacteria, it is first necessary to prepare 

 satisfactory spreads for the purpose. In order that the description 

 shall be as practical as possible, we will suppose that the micro- 

 organisms to be stained are in blood — spiroch^ta, Plasmodium, etc. 



As pointed out above, the protozoa, under such circumstances, 

 are distributed among or in cellular elements that interfere with 

 satisfactory observation unless precautions are taken to separate 

 them as widely as may be required. 



1. Cover-glasses. — The glasses should be perfectly clean and freed from fat, 



either by washing in alcohol and ether and wiping with a clean soft 

 cotton cloth or Chinese rice paper, or by flaming. The drop of blood 

 should be small and should be placed upon the center of one glass and 

 immediately covered by another, so held that the corners do not 

 coincide. As soon as the drop is fairly well distributed the glasses are 

 gently slid apart. 



2. Slides. — The slides, like the cover-glasses, must be perfectly clean. The 



drop of blood is placed upon one slide at about one-fourth the 

 length of the slide from its end, touched with the end (it must have 

 ground edges) of the second slide, and then gently pushed along until 

 the fluid is exhausted. 



If the covers are to be stained, they can most conveniently be 

 held in the Stewart forceps. If the slides are used, they can be 

 held in the fingers. 



