100 



MICROSCOPIC METHODS 



in solution in water have a great tendency to decompose. 

 Only small quantities should therefore be prepared at a time. 



The Staining of Films. — Films are made from cultwes as 

 described above, and a few drops of the stain are placed on 

 the surface. When the preparation has been exposed for the 

 requisite time, usually a few minutes, it is well washed in tap 

 water in a bowl, or with distilled water with such a simple 

 siphon arrangement as that figured (Fig. 38). The figure 

 explains itself. 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 advan- 

 tageous 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 films on 

 slides, a drop of cedar -wood oil is 

 placed on the film directly, and the 

 preparation is then examined. 



Films of fivMs from the body 

 (blood, pus, etc.) can be generally 

 stained in the same way, and this is 

 often quite sufficient 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 1-2 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 

 Romano wsky stains (vide p. 1 1 1) 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 



Fig. 38. — Siphon wash- 

 bottle for distilled water 

 used in washing prepara- 

 tions. 



