66 BACTERIOLOGY 



form a small drop, which is transferred to a cover-glass. 

 A ' hollow ' slide is then taken, that is, one with an exca- 

 vation or well ground in the centre ; this well is surrounded 

 with vaseline by means of a fine hair pencil, and the cover- 

 glass with the drop turned downwards is laid on the slide 

 in such a way as to adhere firmly to the vaseline. If the 

 substance, the microbic contents of which it is desired to 

 examine, is not a liquid containing bacteria, but animal 

 tissue or solid culture medium, a drop of sterile water or 

 sterile salt solution is conveyed on to the cover-glass with 

 the loop of the platinum needle, and a minute sample of 

 the mass to be examined is transferred into the drop. 



In observing the hanging drop the edge must first be 

 sought for with a low power, and then focussed with 

 a higher ; since, as it appears bounded by a sharply- 

 drawn line, the micro-organisms in the drop can in this 

 way be more easily focussed, which very much facili- 

 tates the examination for beginners ; and, morever, the 

 elements are met with in a thinner layer at the border than 

 in the centre. As the elements under observation are not 

 stained, the narrowest possible aperture of the diaphragm 

 must be used in the examination. 



With the hanging drop attention must in like manner 

 be paid to the peculiarities which can be observed in bacteria 

 examined in the fresh state, as detailed above, and their 

 motility is more distinctly brought out in this mode of 

 investigation. The closure of the space prevents the fluid 

 from evaporating, but if the examination is too prolonged 

 the micro-organisms sink into the concavity of the drop, 

 and so sometimes elude observation. 



When it has been ascertained by means of fresh pre- 

 parations that micro-organisms are present, and their form, 

 mode of propagation, and power of movement have been 

 observed, the next step is that of staining. So many pro- 



