68 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



for those objects that are bright enough to be reflected from 

 the unsilvered part of the mirror. 



The Arc and Nonius. The Arc is divided into 90 degrees 

 from the right to the left, and each degree is subdivided 

 into three parts, or 20 minutes, which are again subdivided 

 by the Nonius into every minute. The Nonius is numbered 

 at every fifth of three divisions, from the right to the left, 

 with 5, 10, 15 and 20. The first division to the right hand 

 being to be considered the index division. 



Directions to hold the Quadrant. It is recommended 

 to support the weight of the instrument by the right hand, 

 and reserve the left to govern the index ; place the thumb 

 of the right hand against the edge of ihe quadrant, under 

 the swelling part on which the sight stands, extending the 

 fingers across the back of the quadrant, so as to lay hold on 

 the opposite edge, placing the .forefinger above, and the 

 other fingers below the swelling part, or near the horizon 

 glass ; thus the instrument may be supported conveniently 

 in a vertical position, by the right hand only. By resting the 

 thumb of the left hand against the side, or the fingers 

 against the middle bar, you may move the index gradually 

 either way. 



To adjust the Quadrant. It is a peculiar excellence of 

 the Quadrant that the errors to which it is liable are easily 

 detected and soon rectified ; the observer may, therefore, if 

 he will be attentive, always put his instrument in a fit state 

 for accurate observation. 



1. To adjust the Index Glass, or make it perpendicular 

 to the Plane of the Instrument. Hold the Quadrant in a 

 horizontal position, with the index glass close to the eye, 

 look nearly in a right line down the glass, and in such a 

 manner, that you may see the arc of the quadrant by direct 

 view, and by reflection at the same time. If they join in 

 one direct line, and the arc seen by reflection forms an 

 exact plane with the arc seen by direct view, the glass is 

 perpendicular to the plane of the quadrant ; if not, the 

 error must be rectified by altering the position of the screws 

 behind the frame. 



2. To adjust the Horizon Glass, and set it parallel to the 

 Index Glass. Set the index line of the nonius exactly at o 

 on the limb, and fix it there by the screw at the under side. 

 Now look through the sight at some distant small object ; 

 the object will be seen directly through the unsilvered part 



