102 



MICROSCOPIC METHODS 



difficult to overstain with it. Thionin-blue also gives good dif- 

 ferentiation and does not readily overstain. Its tone is deeper 

 than that of methylene-blue, and it approaches the violets in tint. 

 Bismarck-brown is a weak stain, but is useful for some purposes. 

 Formerly it was much used in photomicrographic work, as it was 

 less actinic than the other stains. It is not, however, needed 

 now, on account of the improved sensitiveness of plates. 



It is most convenient to keep saturated alcoholic solutions 

 of the stains made up, and for use to filter a little into about 

 ten times its bulk of distilled water in a watch-glass. A solution 

 of good body is thus obtained. Most 

 bacteria (except those of tubercle, 

 leprosy, and a few others) will stain in 

 a short time in such a fluid. Watery 

 solutions may also be made up, e.g. a 

 saturated watery solution of methylene- 

 blue or a 1 per cent, solution of 

 gentian-violet. Stains must always be 

 filtered before use ; otherwise there 

 may be deposited on the preparation 

 granules which it is impossible to wash 

 off. The violet stains in solution in 

 water have a great tendency to decom- 

 pose. Only small quantities should 

 therefore be prepared at a time. 



The Staining of Cover-glass Films. 

 Films are made from cultures as 

 described above. The cover-glass may 

 be floated on the surface of the stain 

 in a watch-glass, or the cover-glass held 

 in Cornet's forceps with film side 

 uppermost may have as much stain 

 poured on it as it will hold. 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. 42). 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 advantageous 

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

 films can be subsequently dried and mounted permanently. 



FIG. 42. Siphon wash- 

 bottle for distilled water 

 used in washing prepara- 

 tions. 



