USES OF DIFFERENT STAINS. 



93 



Of the stains specified, the violets and reds are the most 

 intense in action, especially the former. It is thus easy in using 

 them to overstain a specimen. Of the blues, methylene-blue 

 probably gives the best differentiation of structure, and it is 

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

 differentiation 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 the 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 (ex- 

 cept 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 i 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 decompose. 

 Only small quantities should there- 

 fore be prepared at a time. 



The Staining of Cover-glass Films. 

 Films are made from cultures as FIG. 36. Syphon wash- 

 described above. The cover-glass bottle for distilled water used 

 may be floated on the surface of the in washing preparatk 

 stain in a watch-glass for about five minutes, or the cover- 

 glass held in forceps with film side uppermost may have 



