USE OF THE MICROSCOPE xxiii 



of which there is a series, of various sizes, fixed in such a way 

 as to allow of their being easily changed. 



The combination of lenses at the upper end of the body 

 is the eye-piece ; the combination at the lower end is the 

 objective. 



One or two eye-pieces, and two objectives of different 

 magnifying powers, J inch and J inch, are required. 



An object can be seen clearly through a microscope only 

 when it is at a certain definite distance from the objective ; 

 and this distance varies with different objectives and eye- 

 pieces, and to a slight extent with different observers. An 

 objective of high power requires to be nearer to the object 

 than does one of low power. The regulation of this distance 

 is called focussing, and is effected in two ways. 



The coarse adjustment of focus is made by simply sliding 

 the body up or down, with a slight twisting movement, 

 through the tube of the stand in which it is supported, or 

 else by a rack and pinion worked by a milled head. 



The fine adjustment is effected by means of a screw, the 

 position of which differs in different microscopes ; but which 

 should be so placed that it can readily be worked with either 

 hand. By turning the head of the screw from left to right, 

 in the direction of the hands of a watch, the body of the 

 microscope is lowered, and the objective brought nearer to the 

 object : by turning in the reverse direction, the objective is 

 raised. 



In using the microscope the following rules are to be 

 carefully observed. 



1. Examine every object with the low power first. Having 

 adjusted the eye-piece and the objective, turn the mirror so as 

 to reflect the light up the body of the microscope : place the 

 object on the stage under the objective, and carefully lower 

 the body with a screwing motion till the objective is about a 

 quarter of an inch from the cover-glass : then look through 

 the eye-piece, and gradually raise the body till the object 

 becomes distinctly visible. Focus accurately with the fine 

 adjustment screw. With the high power, begin with the 

 objective close to the cover-glass, and then proceed as before. 



