r '02 3 



viation of the vifual ray from a parallelifm to the 

 plane of the quadrant, is to twice an arch equal to 

 the verfe-fme of the deviation; as the tangent of 

 half the angle meafured by the quadrant is to radius, 

 ■very nearly. Thus a deviation of i° in the line of 

 iight, will produce an error of about i / in meafuring 

 an angle of 90 , whether by the fore or back ob- 

 fervation 3 but the fame deviation will produce an 

 error of 4/ in meafuring an angle of 1 50 , of 6' in 

 taking an angle of 160% and 12'' in taking an angle 

 of 17.0 . Hence a pretty exact adjuflment of the 

 line of light, or axis of the telefcope, is requifite in 

 meafuring large angles, fuch as thofe are taken by the 

 back obfervation: and therefore a director of the 

 light ought by no means to be omitted in the con- 

 itruction of the inftrument (as it commonly has been 

 fince Mr. Hadley's time, though recommended by 

 him), except a telefcope be made ufe of, which if 

 rightly placed anfwers the fame purpofe better, efpe- 

 cially in obferving the diftance of the Moon from 

 the Sun between the firil: and laft quarter. The 

 director of the fight may be placed exact enough by 

 •conftruction ; but the telefcope cannot, and Mr. 

 Hadley, not having been aware of the importance of 

 an exact polition of it, has accordingly given no di- 

 rections for the placing it. I mail therefore endea- 

 vour to fupply this defect in the following remarks. 



In the nrft place, I would by all means recommend 

 an adjufling piece to be applied to the telefcope, 

 whereby its axis may be brought parallel to the 

 plane of the quadrant : in the next place, the back- 

 horizon-glafs ought to be nlvered in the fame 

 manner as the fore-horizon-glafs : and thirdly, two 

 thick filver wires fhould be placed within the eye-tube 



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