EXAMINATION OF UNSTAINED PREPARATIONS. 175 



The lens is then unscrewed from the microscope and 

 placed in the case intended for its reception. 



During work, of course, the lens need not be cleaned 

 and put away after each examination ; but when the 

 work for the day is over, an immersion lens must always 

 be protected in this way. Under no circumstances should 

 it be allowed to remain in the immersion oil or exposed 

 to dust for any length of time. 



EXAMINATION OF UNSTAINED PREPARATIONS. 

 ''Hanging drops" It frequently becomes necessary to 

 examine bacteria in the unstained condition. The cir- 

 cumstances calling for this arise while studying the 

 multiplication of cells, the germination of spores, the 

 formation of spores, and the absence or presence of 

 motility. 



In this method the organisms to be studied are sus- 

 pended in a drop of physiological salt solution or bouillon 

 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. 33). The slip is held in posi- 

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



FIG. 33. 



Hollow-ground glass slide for observing bacteria in hanging drops. 



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 times requires con- 



