THE MICROSCOPE AND MICROSCOPIC METHODS 39 



heat would cause no discomfort to the hand. Having dried 

 the preparation, it is to be passed through the flame of a Bunsen 

 burner or alcohol lamp three times, taking about one second for 

 each transit. The heat of the flame serves to dry the bacteria 

 upon the cover-glass and fix them permanently in position; it is 

 not sufficient, however, when applied in this manner, to kill all 

 kinds of bacteria, especially those containing spores. After it 

 has been passed through the flame three times the preparation 

 may be stained with one of the aniline dyes, and after washing 

 in water and drying, may be mounted, face down, in Canada 

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

 amined 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 





Pig. 26. — Kirkbride forceps for holding slides. 



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. Special care should be exercised 

 to avoid soiling the surface of the objective with the material on 

 the slide, for inattention to this matter may permit the confusion 

 of material from two different specimens, a bit being carried from 

 one, smear to another by the lens. The forceps of Kirkbride will 

 be found convenient when staining on the slide. The aluminium 



