20 MICROSCOPY FOR BEGINNERS. 



In the older stands a single small milled head will be 

 found on the front of the body near the lower end, just 

 above the society-screw. In more recent stands it will 

 be on the arm at the back of the instrument. This is the 

 "fine adjustment screw ;" and although it adds somewhat 

 to the cost, it should always be on the stand if the pur- 

 chaser desires to use even moderately high-power objec- 

 tives. For low-powers it is not necessary. The fine ad- 

 justment screw is so made that by turning its milled head 

 the objective, if the adjustment is at the front, or the 

 entire body, if it is at the back, is slowly raised or low- 

 ered. When the high-power objective has been imper- 

 fectly focussed by racking the body upward^ it seldom 

 happens that the image is as distinct as is desirable ; 

 therefore the microscopist, by a few gentle turns of the 

 fine adjustment screw, raises or lowers the objective, un- 

 til the magnified image has its outlines as sharply de- 

 fined as the figures in the best steel engravings. With 

 the one-inch objective, or others still lower (two, three, 

 or even four inch), the focus can be accurately obtained 

 by the coarse adjustment alone, but with the or the 

 fine adjustment must always be used. 



It is a great mistake made by some who ought to 

 know better, to try to examine an object not distinctly 

 in focus. In such cases the strain on the eye is severe 

 and injurious, while the pleasure of examining the 

 preparation is much lessened. The changes made for 

 the better by a few delicate touches of the fine adjust- 

 ment can be appreciated only when seen. Always try 



