r to2 ] 
vi nion o r dr* v ! ! u >1 ray from a parallelism to the 
plane of the q i.Trant, is to 'twice an arch equal to 
the verfe-ilne of the deviation; as the tangent of 
hair the angle meafured by the quadrant is to radius, 
very nearly. Thus a deviation of i° in the line of 
fight, will produce an error of about i' in meafuring 
an angle of 90°, whether by the fore or back ob- 
fervation ; 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 r yo°. Hence a pretty exadt adjustment of the 
line of light, or axis of the telefcope, is requill te in 
meafuring large angles, fuch as thofe are taken by the 
back obfervation : and therefore a diredtor of the 
fight ought by no means to be omitted in the con- 
firudtion 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 firft and laft quarter. The 
director of the fight may be placed exadt enough by 
conftrudtion ; but the telefcope cannot, and Mr. 
Hadley, not having been aware of the importance of 
an exadt pofition of it, has accordingly given no di- 
redtions for the placing it. I fhall therefore endea- 
vour to fupply this defedt in the following remarks. 
In the firft place, I would by all means recommend 
an adjufting 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 filvered in the fame 
manner as the fore-horizon-glafs : and thirdly, two 
thick filver wires fhould be placed within the eye-tube 
in 
