INTRO D UC TOR Y. 9 



verse arm, like a telescope ; this motion is for coarse 

 focussing. For fine adjustment it is moved by a milled 

 head, g, which acts upon a screw inside the pillar d. 

 The joint at h enables the observer to place the instru- 

 ment at any angle he may require. The mirror, for 

 throwing light through transparent objects, is seen at 

 k ; a condensing lens, for throwing light upon opaque 

 objects, is seen at i. So much for the mechanical 

 arrangement. But where are the most important parts 

 of the microscope the lenses, upon the combination of 

 which the magnifying power of the instrument depends? 

 These lenses, which are known by the names of " ob- 

 ject-glasses " and " eye-pieces ;; respectively, fit, the 

 former by a screw, into the bottom of the body, /; the 

 latter, m, by sliding into its top portion. I would 

 advise you to learn the names of these different parts 

 of a compound microscope. Of course, I need hardly 

 tell you that there is great difference in the forms of 

 compound microscopes, their mechanical arrange- 

 ments, and so on; but the above description will 

 seive to give you a fair idea of the general plan of a 

 compound microscope. 



Let us now look a little more closely into the struc- 

 ture of the lenses, on which the magnifying power of 

 the microscope depends. I have already told you that 

 these are known by the names of " eye-pieces " and 

 "object-glasses," or "objectives," as they are some- 

 times termed. The names are easy to remember and 

 explain their respective uses, the former being looked 

 through by the eye of the observer, the latter being 

 placed near the object you wish to examine. The ordi- 

 nary eye-piece consists of two plano-convex* glasses, 

 the plane surfaces of each being directed upwards. 

 That one near the eye is the " eye-glass," the one at 



* A plano-convex lens is one which has one of its surfaces plane 

 or flat, the other convex. 



