UNSTAINED PREPARATIONS. 183 



the examination is finished raise the objective from the 

 preparation by turning the screw of the coarse adjust- 

 ment toward you. Remove the preparation from the 

 stage, and, with a fine silk cloth or handkerchief, wipe 

 very gently and carefully the oil from the face of the lens. 

 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 circum- 

 stances 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 bou- 

 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. 38). The slip is held in 



Fig. 38. 



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 



