25 



tion about the centre of the index. Thus by the screws A and 

 if and the steady pins k and m, the little mirror is made erect 

 arid fixed on the plate A : it is also set at right angs. to the 

 index mirror B, by loosening a little the screws s s s s, and turn- 

 ing the whole plate A by its milled edge, round its centre on 

 either side, so far as necessary ; and when this is found to be 

 accurately effected, the screws sss-s- are to be again made 

 fast; when the little mirror will be perpendicular both to the 

 plane of the instrument, and also to that of the great mirrors 

 and cannot,^ without suffering violence, alter its position.. 



This circular motion of the plate A, and of the little mirror 

 fastened to it, is permitted, without communicating any mo- 

 tion or even contact of it, to the index-glass, its cock, steady 

 pins, or screws, by the following contrivance. 



Through the plate A are cut long holes or slrts, formed as 

 represented in the fig. concentric with it, at the places of 

 the screws s s s s, f, and g, and also at the pins d d and e e . 

 these slits are made just so wide, as that the screws and pins 

 will not touch them, and that the heads of the screws will, 

 when screwed down, press upon the edges of the slits : the 

 slits at e g are not represented in the fig. to avoid confu- 

 sion : the slits at s and g should be so long as to allow the 

 plate A to turn through the space about -~\.\i of an inch on 

 each side of the screws fixed erect; and the slits atdd, e e^ 

 and f, may be shorter, according as they lie nearer the cen-r 

 tre, each slit bounded within the same sector of a circle., 



VOL. XII^ £ 



