SURVEYING INSTRUMENTS. 67 



it is perpendicular to the plane of the instrument; this 

 mirror being fixed *to the index, moves along with it, and 

 has its direction changed by the motion thereof; this glass 

 is designed to receive the image of the sun, or any other 

 object, and reflect it on the horizon glass. The brass frame 

 with the glass is fixed to the index by screws which serve 

 to adjust it in a perpendicular position. 



Of tlie Horizon Glass. On the radius of the frame is a 

 small speculum, the surface of which is parallel to the index 

 glass, when the counting division of the index is at O on 

 the arc, and receives the reflected rays from the object, and 

 transmits them to the observer. -The horizon glass is not 

 entirely quicksilvered, but only on its lower half, or that 

 next to the frame of the quadrant, the other half being 

 transparent ; and the back part of the frame is cut away, that 

 nothing may impede the sight through the unsilvered part 

 of the glass. The edge of the foil of this glass is about pa- 

 rallel to the plane of the instrument, and ought to be very 

 sharp, and without a flaw ; the glass is set in a brass frame, 

 to which there is an axis which passes through the wood- 

 work, and is fitted to a lever on the under side of the 

 quadrant; by this lever the glass may be turned a few 

 degrees on its axis, in order to set it parallel to the index 

 glass ; the lever has a contrivance to turn it slowly. To 

 set this glass perpendicular to the plane of the quadrant 

 there are two sunk screws, one before and one behind the 

 glass ; these screws pass through the plate on which the 

 frame is fixed, into another plate, so that by loosening one, 

 and tightening the other of these screws, the direction of 

 the frame, with its mirror, may be altered, and thus set 

 perpendicular to the plane of the instrument. 



Of the Shades. These are two red, or dark, and one 

 green glass ; they are used to prevent the rays of the sun 

 from hurting the eye at the time of observation ; they are 

 each of them set in a brass frame, which turns on a centre, 

 so that they may be used separately, or together, as the 

 brightness of the sun may require. These glasses are fixed 

 on the frame, between the index and the horizon glasses. 



Of the Sight Vane. This is a piece of brass fixed on the 

 frame opposite the horizon glass, perforated with two small 

 holes, one exactly at the height of the quicksilvered edge 

 of the horizon glass ; the other somewhat higher, to direct 

 the sight to the middle of the transparent part of the mirror 



