THE MICROSCOPK AND ITS PARTS. I9 



in a joking way, "that nothing will throw a microsco- 

 pist into a chill more quickly than to see a friend look 

 into his microscope and focus down with the coarse ad^ 

 justment." Yet men who should know better have 

 been seen to do this reprehensible thing. 



In the older instruments 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 instru- 

 ment. This is the "fine-adjustment screw;" and al- 

 though it adds somewhat to the cost, it should always- 

 be on the stand if the purchaser desires to use even 

 moderately high-power objectives. For low-powers it 

 is not necessary. The fine-adjustment 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 lowered. When the 

 high-power objective has been imperfectly focused by 

 racking the body upward, it seldom happens that the 

 image is as distinct as desirable ; therefore the microsco- 

 pist, by a few gentle turns of the fine-adjustment screw, 

 raises or lowers the objective, until the magnified image 

 has its outlines as sharply defined as the figures in the 

 best steel engravings. With the one-inch objective, 

 or with 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 \ the 

 fine. adjustment must always be used. 



A mistake often made by some who should know 

 better, is to try to examine an object not dis- 

 tinctly in focus. In such cases the strain on the eye 

 is severe and injurious, while the pleasure of observing 

 the preparation is much lessened. The changes 



