EXAMINATION OF BACTERIA WITH THE MICROSCOPE. 27 



makes a suitable object to be examined with the oil-immersion 

 objective. 



The smear preparation may equally well be made directly 

 upon the glass slide. The fixation in the flame must then occupy 

 a longer time than with the small and thin eover-glass. Such 

 preparations have the advantage that several may be made upon 

 one slide, and that after staining them they may be examined in 

 cedar-oil, with the oil-immersion lens, without the use of the 

 cover-glass and Canada balsam. The forceps of Kirkbride will 

 be found convenient for staining on the sHde. Experiments per- 

 formed in the writer's laboratory have shown that the ordinary 

 method of fixation in the flame, when applied to bacteria spread 



Fig. 9. — KiEKBRiDE Forceps por Holding Slides. 



upon shdes, has httle effect on the vitality of many species. The 

 beginner is, therefore, advised to make his preparations on 

 cover-glasses. 



When very resistant or dangerous pathogenic bacteria are 

 being handled, after fixation by heat upon the slide or cover- 

 glass, the preparation may, if desired, be immersed in i-iooo 

 solution of bichloride of mercury long enough to kill the bacteria, 

 without injuring the preparation or interfering with its staining 

 properties. 



Staining. — The bacterial cells are devoid of color, and the 

 object of staining them is to give them artificially some color 

 that would make them distinct and easily visible with the micro- 

 scope. In order that they shall stand out sharply the stain 



