186 BACTERIOLOGY. 



illon in the centre of a cover-slip. This is then placed, 

 drop down, upon a slide, in the centre of which a hollow 

 or depression is ground (Fig. 35). The slip is held in 



FIG. 35. 



Longitudinal section of hollow-ground glass slide for observing bacteria in 

 hanging drops. 



position by a thin layer of vaselin, which may be 

 painted around the margins of the depression. This 

 not only prevents the slip changing 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 indispen- 

 sable for the purposes mentioned, and at the same time 

 requires considerable care in its manipulation. The 

 fluid is so transparent that the cover-slip is often broken 

 by the objective being brought down upon the prepara- 

 tion before one is aware that the focal distance has been 

 reached. This may be avoided by grasping the slide 

 with the left hand and moving it back and forth under 

 the objective as it is moved 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 rendered 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, when the edge of the drop 

 is readily detected with the higher power lens some- 

 where near the centre of the field. 



In examining bacteria by this method there is a pos- 



