MICROSCOPIC EXAMINATIONS. 109 



centre of a cover-slip. This is then placed, drop down, 

 upon an object-glass in the centre of which a hollow or 

 depression is ground (Fig. 20). The slip is held in posi- 

 tion by a thin layer of vaselin, which may be painted 



FIG. 20. 



around the margins of the depression. This not only 

 prevents the slip from moving from its position during 

 examination, but also prevents drying by evaporation if 

 the preparation is to be observed for any length of 

 time. This is known as the " hanging drop " method of 

 examination or cultivation. It is indispensable for the 

 purposes mentioned, and at the same time requires con- 

 siderable care in its manipulation. The fluid is so trans- 

 parent that the cover- slip is often broken and the face 

 of the objective injured by its being brought down upon 

 the preparation before one is aware that the focal dis- 

 tance has been reached. This may be avoided. by grasp- 

 ing the slide with the left hand and moving it back 

 and forth under the objective as it is brought down 

 toward the object. As soon as the least pressure is felt 

 upon the slide the objective must be raised, otherwise 

 the cover-slip will be broken and the lens may be ren- 

 dered worthless. 



A safer plan is to bring the edge of the drop into the 

 centre of the field with one of the higher power dry 

 lenses. When this is accomplished, substitute the im- 

 mersion for the dry system, and the edge of the drop 

 can now easily be found. 



6 



