20 



Through these sHts in the plate A, the screws s s s s, f, and 

 ^ g are inserted, and all except g fastened in the head of the 

 index i in which latter, the pins e e penetrate also through 

 the wing D of the cock of the index-glass, to steady it. 



^ The edge of the index-glass D rests on the pins d d, which 

 project only so ftir above the surface of the plate J, as to 

 keep the nHr^ and the wing D of its cock clear of it, so as 

 that the plate can turn about under both without touching 

 tliem : and the bar or wing E of the cock c lies about |th of 

 an inch above the wing D, so as to be quite clear of it, and 

 permit adjustment by raising or depressing the wing. It may 

 be supposed that the cock c and its wing E must at first be 

 so formed that the mirror C, when fastened to them, will be 

 nearly at right angles to the mirror J3, when the bar E lies 

 parallel to the index mirror, and the screws, &c. are in the 

 middle of the slits in the plate A ; that a small metion of the 

 plate A on either side, will suffice for an exact adjustment of 

 the mirror C, 



There is a round hole in the middle of the plate A, a good 

 deal wider than the head of the pin, about which the index 

 turns ; and the plate is made to turn concentric with the pin 

 by a little ring or sock'.'t R, brazed in the hole in the plate, 

 or by an annular ledge formed on it projecting downward 

 below the under surface of the plate about TTth part of an 

 inch. The outside of this projecting ring is to be exactly 

 fitted into a circular groove or cavity formed in the index, so 



