MIRROR. 3 



1-inch, |-inch, |-inch object-glass, &c. The object- 

 glasses, for brevity, are often called powers. 



As a beginner may at first have some difficulty in 

 distinguishing a high from a low power, it may be 

 remarked that the size of the lower glass is larger 

 the lower the power : but, in the case of the better 

 object-glasses, the focal distance is engraved on the 

 box in which the object-glass is packed when put 

 away. 



As the coarse movement raises or depresses the 

 body and object-glass through comparatively large 

 distances, it must be used only with the lower object- 

 glasses, i. e. those of low or little magnifying power, 

 as the 2-inch, 1-inch, or |-inch, or to bring the 

 object-glass near the focal distance with the higher 

 powers ; whereas the fine movement serves to adjust 

 the higher powers, as the J-inch, &c. 



If the object-glasses should become soiled on the 

 lower face, this should be wiped very gently with an 

 old silk handkerchief or piece of very soft wash- 

 leather, previously shaken to displace dust. The 

 same method will answer to cleanse the upper surface 

 of the eye-piece. 



Great care must be taken that a slide which has 

 been warmed in any experiment be not placed near 

 the object-glass until quite cold. 



Mirror. The mirror has sometimes one silvered 

 face only, at others two one flat, the other concave. 

 The flat surface is used to reflect the light upon the 

 object when the light is too great with the concave 

 surface. 



Beneath the stage, in most microscopes, is a cir- 

 cular moveable ' { diaphragm," perforated with holes 

 of various sizes, to allow more or less of the light 

 reflected by the mirror to pass through, as may be 

 required. 



When opake objects are viewed, the mirror should 



