16 MICROSCOPIC EXAMINATION OF BACTERIA. 
a ‘concave slide ;’ or use a slide on which is 
cemented a small glass ring, and vaselin the top 
of the ring. 
(6) On “the centre of a clean cover-glass place a 
small drop of water. 
(c) With a sterile platinum needle add to the 
drop of water a very small portion of the culture 
to be examined. 
(d) Invert the slide over the cover-glass, so that 
the drop of water is covered by the concavity or is 
inside the glass ring, but does not touch the sides 
of either ; press down so that the chamber is sealed 
tight by the vaselin. 
(e) Invert carefully and examine. 
The hanging-drop examination is for the purpose 
of determining the motility of bacteria or for watch- 
ing their reproduction. The preparation may be 
kept for examination from day to day without loss 
by evaporation. 
IV. EXAMINATION OF STAINED BACTERIA. 
1. Ordinary Stains. 
(a) Prepare a clean cover-glass.' 
(4) Place a drop of water on the glass.’ 
(ce) With a sterile platinum needle transfer a 
minute portion of a culture to the drop of water 
1 See footnote, page 14, for directions for cleaning covers. 
2 Tf the cultures are in bouillon or other fluid, it is often 
unnecessary to use the drop of water in spreading them on 
the cover-glass. 
