THE FINE ADJUSTMENT 51 



shorter than the Continental, but is provided with a 

 draw tube which, when fully extended, gives a tube 

 length greater than the English, thus permitting the 

 use of either form of objective. 



For practical purposes the tube length = distance from the 

 end of the nosepiece to the eyeglass of the ocular. This is the 

 measurement referred to in speaking of "long" or "short" tube. 



FIG. 42. Coarse adjustment. 



FIG. 43. Fine adjustment. 



3. The coarse adjustment (Fig. 40, c) should be a 

 rack-and-pinion movement, steadiness and smoothness 

 of action being secured by means of accurately fitting 

 dovetailed bearings and perfect correspondence between 

 the teeth of the rack and the leaves of the pinion (Fig. 

 42). Also provision should be made for taking up the 

 "slack" (as by the screws A A, Fig. 42). 



4. The fine adjustment (Fig. 40, d) should on no 

 account depend upon the direct action of springs, but 



