14 • 



for both these, since the incidence of the rays from the 

 middle, and of those from the normal edge of the index 

 glass on the horizon glass, is different ; so the position of the 

 back sight vane, and the position and direction of a tele- 

 scope, (if one be used,) must be altered for these different 

 purposes. The vane may, without moving its support, have 

 its position changed, by having the eye hole.made in a little 

 'moveable plate fastened on the support ; but a complicated 

 motion would be requisite for the telescope, to place it in 

 the best manner for each of the above intentions. If it is 

 expected to answer by only a circular motion of its upright 

 stand, changing its direction, without moving it from its 

 place, the stand should be placed as near as possible to the 

 back horizon glass ; for the farther it is removed from it, the 

 more distant in one of its two positions will its axis be from 

 the axis of that glass. 



To ascertain the direction of the sight and of the telescope 

 in making an observation by the edge of the .index-glass, 

 or of the glass here to be proposed for. the same purpose, let 

 a moveable rule or square, perpendicular to the face of the 

 quadrant, be applied to the farther side of the quadrant 

 opposite to the back horizon glass; and when the direct and 

 reflected images are brought to unite, as the eye looks 

 through the axis of the glass, let the rule be shifted, till its 

 edge is made to appear in the place of their coincidence. ^If 

 then a mark be made on the side of the quadrant at the edge 

 of the rule, a line drawn from the mark through the axis of 



