( 395 j 
XXVII. A general Theory for the Menfuration of the Angle fub-- 
t ended by Two Objects, of which One is obf erased by Rays after 
T wo Reflections from plane Surfaces , and the other by Rays • 
coming directly to the Spectator s Eye., By George Atwood 3 . 
M^A. Ft Rt So 
Read' June 21, 1781* 
T HE a£hial determination of an ' angle implies two ohfer- ■ 
vations,, one taken at each extremity of the arc by 
which that angle is meafured. When fixed agronomical qua-- 
drants or other feftors are ufed for the pradl ical eflimation of 
angles* one of thefe obfer vations is previoufly, made by diredt- 
ing the axis of the telefcope or line of collimation to forne fixed - 
point in the heavens, the index being then coincident with the 
initial point on the arc of the feftor : after this adjufhnent : 
one, obfervation only is neceffary to afcertain the angular. 
diflance- 
