xxvi INTRODUCTION. 



is a tube of metal carrying the lenses, which are the essential 

 part of the instrument. 



The stand supports the body in such a way that the latter 

 may be moved up and down without any lateral movement. 

 It also supports a stage, which ought to be horizontal, to 

 hold the object to be examined. The stage has a round 

 hole in its centre, through which light may be transmitted 

 through the object by a mirror fixed below the stage. This 

 aperture is usually much larger than is necessary ; but the 

 excess of light, and especially the light far from the optical axis 

 of the lenses, is cut off either by means of diaphragms, of which 

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

 allow of their being easily changed, or by an ' iris-diaphragm.' 



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, \ inch or low power, and or -| inch or 

 high power, 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 lower 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 jine 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 micro- 

 scope is lowered, and the objective brought nearer to the 

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

 raised. 



