MICROSCOPICAL EXAMINATION OF BACTERIA. 87 



off witli distilled water by means of a siphon apparatus or a wash- 

 bottle. The cover-glass may be allowed to dry, and then mounted 

 in Canada balsam, or it may, while still wet, be turned over on to a 

 slide, the excess of water removed with filter-paper, and the exposed 

 surface wiped dry. It may first be examined with a power of about 

 250 diams. ; and if a high magnification is required, which is usually 

 the case, a droplet of cedar oil is placed on the cover-glass, and the 

 specimen examined with an immersion lens. 



If the specimen is to be made permanent, fix the cover-glass at 

 one corner with the thumb, and with a soft rag carefully wipe off 

 the cedar oil ; then float off the cover-glass by running in distilled 

 water at its margin, and having made a little ledge with a strip of 

 filter-paper, place the cover-glass up against it upon one of its 

 edges and leave it to dry. When perfectly dry mount in Canada 

 balsam, or put it away in a cover-glass box provided with a label of 

 contents. 



In many cases it is necessary or preferable to apply the stain 

 for a much longer period. This may best be effected by pouring 

 some of the staining solution into a watch-glass, and allowing the 

 cover-glasses to swim on the surface, with their prepared side, of 

 course, downwards. Throughout all these manipulations it is 

 necessary to bear in mind which is the prepared surface of the 

 cover-glass. 



Instead of using the watery solutions of the aniline dyes the 

 author prefers in many cases to use stronger solutions, and to reduce 

 the staining by a momentary immersion in alcohol. Very beautiful 

 preparations of streptococci, sarcinse and other bactei'ia can be 

 obtained by this method, which is as follows : Cover-glass prepara- 

 tions are stained with carbolised fuchsine (Neelsen's solution) for 

 about two minutes, rinsed in alcohol for a few seconds, quickly 

 washed in water, and either examined in water or dried and 

 mounted in the usual way. The extent of decolorisation is a 

 m.atter of practice : a momentary immersion in alcohol is sometimes 

 sufficient ; too long immersion will remove too much of the colour ; 

 too short immersion will leave the dehcate outlines indistinct. This 

 method is especially valuable for sarcinse and streptococci, the 

 divisions between the elements being sharply defijied, and as any 

 albuminous particles or debris in the preparation are decolorised, 

 much cleaner and sharper preparations are obtained than with the 

 watery solutions. Lbffler's and other concentrated solutions may 

 also be used, but Neelsen's solution may be regarded as the standard 

 one for this method. 



