UNSTAINED PREPARATIONS. 185 



one or two turns, the object will be in focus. Do not 

 remove the eye from the instrument until this has been 

 accomplished. 



Then, with one hand upon the fine adjustment and 

 the thumb and index finger of the other hand hold- 

 ing the slide lightly by its end, it may be moved about 

 under the objective. At the same time the screw 

 of the fine adjustment must be turned back and forth, 

 so that the different planes of the preparation may be 

 brought into focus one after the other. In this way the 

 whole section or preparation may be inspected. When 

 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. 



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- 



