96 



MICROSCOPIC METHODS 



When the film has been washed the surplus of water is drawn 

 off with a piece of filter-paper, the preparation is carefully dried 

 high over a flame, a drop of xylol balsam is applied, and the 

 cover-glass mounted on a slide. It is sometimes advantageous 

 to examine films in a drop of water in place of balsam. The 

 films can be subsequently dried and mounted permanently. In 

 the case of tubercle, special stains are necessary (p. 100), but with 

 this exception, practically all bacterial films made from cultures 

 can be stained in this way. Some bac- 

 teria, e.g. typhoid, glanders, take up the 

 stains rather slowly, and for these the 

 more intensive stains, red or violet, are 

 to be preferred. 



Films of fluids from the body (blood, 

 pus, etc.) can be generally stained in the 

 same way, and this is often quite suffi- 

 cient for diagnostic purposes. The blue 

 dyes are here preferable, as they do not 

 readily overstain. In the case of such 

 fluids, if the histological elements also 

 claim attention it is best first to stain 

 the cellular protoplasm with a one to 

 two per cent watery solution of eosin 

 (which is an acid dye), and then to use 

 a blue which will stain the bacteria and 

 the nuclei of the cells. The Roman owsky 

 stains (v. p. 105) are here most useful, as 

 by these the preparations are fixed as 

 well as stained. Fixation by heat which 

 is apt to injure delicate cellular structures 

 is thus avoided. In the case of films 

 made from urine, where there is little 

 or no albuminous matter present, the 

 bacteria may be imperfectly fixed on 



the slide, and are thus apt to be washed off. In such a case it 

 is well to modify the staining method. A drop of stain is 

 placed on a slide, and the cover-glass, film-side down, lowered 

 upon it. After the lapse of the time necessary for staining, 

 a drop of water is placed at one side of the cover-glass and a 

 little piece of filter-paper at the other side. The result is that 

 the stain is sucked out by the filter-paper. By adding fresh 

 drops of water and using fresh pieces of filter-paper, the speci- 

 men is washed without any violent application of water, and 

 the bacteria are not displaced. 



FIG. 44. Syphon wash- 

 bottle for distilled water 

 used in washing prepara- 

 tions. 



