THE COMPOUND MICROSCOPE 



399 



usually consists of two large milled heads (C.) on either side of 

 the limb, and by turning either of these, which actuate a rack 

 and pinion mechanism, a relatively large movement is brought 

 about. The fine ad- 

 justment is generally 

 operated by a single 

 milled head (F.), 

 situated at the top 

 of the limb, and by 

 this means a very 

 slight movement is 

 effected, enabling 

 great accuracy of 

 focussing to be at- 

 tained. 



Below the stage 

 is fixed an adjust- 

 able aperture, the 

 diaphragm (D.), by 

 means of which the 

 amount of light 

 reaching the object 

 on the stage can 

 be regulated. If a 

 condenser is pre- 

 sent, it is placed 

 between the dia- 

 phragm and the 

 stage, and, in the 

 best instruments, its 

 distance below the 

 stage is adjustable 



FIG. 230. Diagrammatic representation of a 

 compound microscope. C., Coarse adjust- 

 ment ; Co., condenser ; D., diaphragm ; D.t., 

 draw-tube ; F., fine adjustment ; Fo., foot ; 

 H., milled head controlling substage adjust- 

 ment for condenser ; L., pillar ; M., mirror ; 

 N., nosepiece ; Ob., objective ; Oc., ocular or 

 eyepiece ; S., stage ; T., body tube. 



by means of another 



milled head (H.). 



The mirror (M.), 



which is concave on 



one surface and flat on the other, is either attached to the 



underside of the stage or (as in the type illustrated) forms part 



of the adjustable system bearing the condenser. 



