1882.] 



MICEOSCOPICAL JOUEKAL. 



11 



deep groove to receive the rack ; the 

 sides of this groove are undercut 

 with secondary grooves at right angles 

 with the first. 



A rib considerably longer than the 

 slide on the limb is attached to the 

 tube. This rib fits upon the slide ; 

 from its centre projects a secondary 

 rib, which bears the rack. On each 

 side of the rib which holds the rack 

 are two small ribs, which fit into the 

 V-shaped grooves of the slide ; they 

 are the bearing surfaces of the tube. 

 From each side of the secondary rib 

 which carries the rack, project two 

 flanges, which fit into th€ under-cut 

 grooves in the deep groove in the 

 centre of the slide. Their purpose is 

 to keep the small ribs closely applied 

 to the V-shaped grooves. 



The fine adjustment is behind. A 



piece about three inches long is cut 

 out from the centre of the slide, 

 which contains the pinion. A small 

 piece is cut off from the end of this 

 piece, so as to allow a motion of 

 about an eighth of an inth. A strong 

 spiral spring is placed above this to 

 keep it pressed down to the bottom 

 of its slot. It can be raised by a lever 

 which is acted upon by the milled- 

 head of the fine adjustment. By turn- 

 ing this screw the lever acts upon the 

 piece containing the pinion and 

 moves it, and with it also the 

 tube. The milled-head is provided 

 with an index and subdivided in such 

 a way that turning the screw past 

 one division will move the tube one- 

 thousandth of an inch. The useful- 

 ness of this device is still further in- 

 creased by a scale subdivided into 



P^ ^S^ 



Fig. 2. — Stage ok the "Congress" Stand. 



