THE MICROSCOPE AND MICROSCOPIC METHODS 



39 



balsam upon a glass slide. It makes a suitable object to be 

 examined with the oil-immersion objective. The slide is a thin 

 slip of glass, 3 inches by i inch, with ground edges. 



The smear preparation may equally well be made directly 

 upon the glass slide provided this be cleaned and heated to insure 

 a clean surface free from oily matter. The fixation in the flame 

 must then occupy a longer time than with the small and thin 

 cover-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. They 

 are also less readily broken in handling. The forceps of Kirk- 

 bride will be found convenient when staining on the slide. The 



FIG. 26. Kirkbride forceps for holding slides. 



aluminium dish devised by Krauss, 1 or some similar dish, will 

 be found useful when the stain has to be heated. Experiments 

 have shown that the ordinary method of fixation in the flame, 

 when applied to bacteria spread upon slides, has little 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 its staining properties. 



Staining Solutions. The staining of bacteria is done for the 

 most part with the aniline dyes. The object of staining bacteria 

 is to give them artificially some color which makes them distinct 



1 Krauss, Jour. A.M. A., Apr. 6, 1912, Vol. LVIII, p. 1013. 



