THE HANGING-DEOP. 143- 



tory, because the liquid soon begins to evaporate and, as a 

 result, rapid currents are established. To overcome this 

 evaporation, it is customary to examine the material in a 

 "hanging-drop." For this purpose, a thick glass slide, hav- 

 ing a concave well in the middle, is made use of. The or- 

 dinary "concave," or hollow slides, usually on the market, 

 are so shallow that the drop of water will touch the bottom. 

 This should not occur. 



The "hanging-drop" is easily made in the following 

 manner: Place a small drop of water on a ^inch, clean 

 cover-glass which is placed on the table or on a short 

 board. The drop must be small enough so that it will 

 not run, if the cover-glass is placed on edge. The growth 

 from the potato, or other medium, is then touched off into the 

 drop of water, by means of a sterile platinum wire. A ring 

 of vaselin is placed around the edge of the well on the upper 

 side of the concave slide, by means of a brush or match 

 stick. The slide, with its ring of vaselin, is then inverted 

 over the cover-glass and gently pressed down The cover- 

 glass now adheres to the slide, which is then inverted. 

 Care should be taken to see that the vaselin is continuous 

 around the edge of the well. If .iuch is the case, no 

 evaporation of the drop of water can take place, and hence, 

 the hanging-drop can be examined at leisure and without 

 the presence of annoying currents in the liquid. Fig. 21 

 indicates the hanging-drop in longitudinal cross-section. 



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Fig. 21. The " hangine-drop," showing 

 concave slide and suspended drop. 



The Ranvier slide is very useful for certain purposes. 

 The central portion of the slide is ground down so that it is- 

 about ^ mm. below the surface. This shallow well is sur- 

 rounded by a deeper groove. The drop of water, instead of 

 being convex as above, is, in this case, flattened out into a thin 

 layer. The edge of the well is ringed with vaselin as above. 



