THE MICROSCOPE 



119 



PRACTICAL DIRECTIONS. 



General Structure of the Compound Microscope, The com- 

 pound microscope, with which you must now become acquainted, 

 consists of a strong stand (Fig. 37, a) 

 from which rises a vertical pillar (b}. 

 To the latter are attached a horizontal 

 plate or stage (c), perforated in the cen- 

 tre with an aperture (d), the size ot 

 which can be varied by means of a 

 diaphragm : an adjustable mirror (e\ 

 placed below the stage : and a vertical 

 tube (f) attached above the stage by a 

 horizontal arm. Two combinations of 

 lenses are used : an objective or object- 

 glass (/$), consisting of a metal tube with 

 two or more lenses fixed into it, which 

 screws into the lower end of the tube : 

 and an ocular or eye-piece (z), consisting 

 of a metal cylinder with a lens at each 

 end, which slides into the upper end of 

 the tube. It is this arrangement of 

 lenses which forms the essential feature 

 of the compound microscope : the ob- 

 ject, placed on the stage, is magnified 

 by the objective, and the magnified 

 image, thrown into the interior of the 

 tube, is further enlarged by the ocular. 



The object is brought into focus i.e., 

 placed at such a distance from the 

 objective that a perfectly clear and well- 

 defined image is obtained in one of 

 two ways. The tube can be raised or 

 lowered either by sliding it up and 

 down in an outer tube or collar (), or, 



in the more expensive instruments, by a rack and pinion : this move- 

 ment forms the coarse adjustment. In addition, all good microscopes 

 have a fine adjustment ', usually consisting of a spring concealed in the 



FIG. 37. Diagram of compound 



microscope. 



a, stand ; b. pillar ; b' '. movable 

 portion of pillar, raised and 

 lowered by fine adjustment ; 

 c. stage ; d. aperture in stage ; 

 e. mirror ; f. tube ; f . milled 

 ring for raising and lowering 

 tube ; g. collar ; h. objective ; 

 i. ocular ; k. screw of fine 

 adjustment. 



