108 BACTERIOLOGY. 



proceed more slowly in the same direction, and, after one 

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

 the eye from the instrument until this has been accom- 

 plished. 



Then, with one hand upon the fine adjustment and 

 the thumb and index finger of the other hand holding 

 lightly the slide by its end, the slide 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 levels of the preparation may one after 

 the other be brought into focus. 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. 

 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 circum- 

 stances calling for this arise while studying the multipli- 

 cation 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 salt solution or bouillon in the 



