( 295 j 
XXVII. A general Theory for the Menfuration of the Angle fob- - 
tended by Two Objects, of which One is obferved by Rays after 
Two Reflections from plane Surfaces , and the other by Rays 
coming directly to the Spectator's Eye . By George Atwood*, 
M,A. F. Ro .So 
Read June 2i ? 1 781* 
HE adtual determination of an angle implies two obfer- 
1 vations, one taken at each extremity of the arc by 
which that angle is meafured. When fixed aftronomical qua- 
drants or other fedtors are ufed for the practical eftimation of 
angles, one of thefe obfervations is previoufly made by diredt- 
ing the axis of the telefcope or line of collimation to fome fixed 
point in the heavens, the index being then coincident with the 
initial point on the arc of the fedtor : after this adjufiiment 
one obfervation only is jaeceffary to afcertain the angular. 
diftance 
