THE COMPOUND MICROSCOPE 



a considerable upward or downward movement to 

 the tube carrying the objective and focussing at 

 once becomes a more simple matter. The coarse 

 adjustment is only useful for examining objects with 

 a low magnification; if we use it when objects are 

 being highly magnified we run the risk of screwing 

 our objective down upon our object, to the certain 

 destruction of the latter and the probable injury of 

 the former. To obviate such a catastrophe, most 

 of the better class microscopes are also provided 

 with a fine adjustment. By means of this adjust- 

 ment, which externally takes the form of a single 

 milled screw, a considerable turn of the screw in 

 either direction only imparts a very slight upward 

 or downward movement to the microscope tube. 

 In the best instruments, movements of as little as 

 one hundredth part of a millimetre may be imparted 

 to the tube by the fine adjustment and, seeing that 

 there are about twenty-five and a half millimetres 

 to the inch, it is obvious that a good fine adjustment 

 is very delicate and, being so, must be treated with 

 care. The fine adjustment is used to supplement 

 coarse adjustment in the final focussing, when using 

 high magnifications. 



A few words may be devoted to the mirror, for 

 on its intelligent use much depends. Usually we 

 shall find that it is plano-concave, that is to say, 

 flat on one side and hollowed out on the other. 

 The use of the mirror, as we have mentioned already, ^ 

 is to reflect rays of light through the opening of the t^-' 

 stage on to the object we desire to examine. Both 



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