136 BACTEEIOLOGY. 



when at work, because the lenses have been corrected for 

 this distance. A deviation from this distance will cause 

 deterioration in the image perceived. Usually, the micro- 

 scope is provided with a nose-piece, which is 10 mm. thick. 

 When, therefore, the nose-piece is attached to the micro- 

 scope, the draw-tube should be raised to the line marked 

 16 (160 mm.) 



The objective is always moved to, or from, the object 

 by means of the coarse adjustment. This is a rack and pin- 

 ion, and is used to bring- the objective near the focus. It is 

 not used for "focussing," except when the low objective 

 (No. 3) is used. When the high power objective is brought 

 as near to the focus as possible, by means of the coarse ad- 

 justment, it is then focussed exactly, by means of the fine 

 adjustment. The latter is a fine micrometer screw, by means 

 of which the entire body of the microscope is raised or low- 

 ered. 



For bacteriological purposes, the microscope should as 

 a rule be kept in a vertical position. It is manifestly in- 

 convenient to examine plates, hanging-drops, etc. , with an 

 inclined instrument. Although a triple nose-piece is not 

 necessary, yet it is a very desirable, time-saving addition 

 to a microscope. 



Care of the microscope. — There is no instrument placed 

 in the hands of the student which requires as careful atten- 

 tion. An untidy microscope at once characterizes the work 

 of the individual. The following general directions should 

 be closely observed in taking care of the instrument: 



1. — The microscope should always be taken up and car- 

 ried about by means of the pillar below the level of the 

 stage. A chamois skin should be employed, in order not 

 to soil or remove the lacquer. It should never be lifted by 

 taking hold of the fine adjustment tube, or by the barrel. 



2. — All unnecessary contact of the fingers with the lac- 

 quered parts should be avoided. If finger-marks have been 



