HANGING DROP 79 



4. Obtain a small quantity of the material it is 

 desired to examine, in the manner detailed above (pages 

 74-76, steps 2 to ii must be followed in their entirety 

 and with the strictest exactitude whenever tube contents 

 are being handled), and mix it with the drop of water 

 on the cover-slip. 



5. Raise the cover-slip in the points of the forceps 

 and rapidly invert it on to the ring cell of the hanging- 

 drop slide, so that the drop of fluid occupies the centre 

 of the ring. (Carefully avoid contact between the 

 drop of fluid and either the 'ring cell or the layer of 

 vaseline. Should this happen, the now infected hang- 

 ing-drop slide and its cover-slip must be dropped into 

 the pot of lysol and a new preparation made.) 



6. Press the cover-slip firmly down into the vaseline 

 on to the top of the ring cell. (This spreads out the 

 vaseline into a thin layer, and besides ensuring the 

 adhesion of the cover-slip, seals the cells and so retards 

 evaporation.) 



7. Examine microscopically. 



The examination of a "fresh" specimen or a "hang- 

 ing-drop" preparation is directed to the determination 

 of the following data : 



1. The nature of the bacteria present e. g., cocci, 

 bacilli, etc. 



2. The purity of the cultivation; this can only be 

 determined when gross morphological differences exist 

 between the organisms present. 



3 . The presence or absence of spores ; when present, 

 spores show their typical refrangibility exceedingly 

 well by this method. 



4. The presence or absence of mobility. In a hang- 

 ing-drop specimen some form of movement can prac- 

 tically always be observed, and its character must be 

 carefully determined by noting the relative positions 

 of adjacent micro-organisms. 



(a) Brownian or molecular movement. Minute par- 



