Tiie Microscope. 139 



or of a bacterial colony, by means of a previously heated plati- 

 num needle, on the center of two cover-glasses (.004-.005), and, 

 laying one cover-glass over the other, squeeze them gently to- 

 gether between the fingers, so that the bacteria are diffused over 

 their surface in a uniform layer. Then slip them apart and 

 leave them to dry. After a few minutes, pass the cover-glasses 

 rapidly through the flame of a spirit lamp, so as to fix the bac- 

 teria on the glass. Cover-glasses so charged may be kept for 

 an indefinite time ; and if an interesting growth of bacteria is 

 met with, it is always well to preserve some specimens in this 

 way for future reference. 



For all practical purposes it may be said that bacteria of 

 which micro-photographs are to be taken must be stained with 

 a red or brown dye, and by far the best medium is a saturated 

 watery solution of vesuvine or Bismarck brown, which should 

 be filtered before use. For precise work, it is always best to use 

 a freshly-made solution, inasmuch as bacteria, torulse, and fungi 

 develop in it after it has been made some time, and their pres- 

 ence may give rise to an erroneous observation. 



The cover-glass, with the bacterial surface downwards, 

 should be floated on the top of the solution, and allowed to re- 

 main there for approximately an hour, though experience will 

 show that the time required will vary according to the speci- 

 men of dye which is used. It is then removed from the solu- 

 tion, washed in water, and afterwards in a half or one per cent, 

 solution of acetic acid, and allowed to dry — or, if time is an ob- 

 ject, pressed between folds of blotting paper — and its surface 

 brushed with a camel-hair brush to free it from particles of dust, 

 etc. It is then mounted in Canada balsam, and is ready for 

 microscopical examination. The best form of balsam for mount- 

 ing bacteria is that which is dissolved in xylol, which is very 

 easy to work with, and does not abstract the dye from the bac- 

 teria. If the slide is to be examined with an oil immersion ob- 

 jective, the edge of the cover-glass should be sealed, when the 

 balsam is dry, with Hollis' glue. If the various stages of the 

 staining process have been properly carried out, the bacteria 

 should appear deeply stained and sharply defined. 



Turning next to a consideration of bacteria as they occur in 

 the organs and tissues of the body, I may cite, as examples, the 



