414 Mr. atwood’s Theory for the Menfuration 
fake of conftruCting an example to this theory, the magnitude 
of the reflecting planes and the angle of incidence on the fixed 
fpeculum B depending on it may be aflumed of the value men- 
tioned in this article. 
1 8. The magnitude of the arc KF, or of the inclination of 
the reflecting planes to the plane of motion is limited by the 
angles which the obferved objeCts fubtend (art. 16.). Fig. 12. 
Becaufe the greateft angle obfervablc will be meafured by four 
times the arc KF, it follows, that the arc KF muft not be lefs 
than one fourth part of the greateft angle intended to be ob- 
ferved by this conftruCtion ; if the inclination denoted by the 
arc KF be fixed at ic/, four times that angle being 40' will be 
greater than the apparent diameters of the fun or any of the 
planets. 
19. It remains to infer from the preceding conftruCtion (fig. 
2), the aCtual meafure of the angle fub tended by the objeCt s 
obferved. This muft be effected by computation, which will 
not only ferve as an illuftration of the theory, but afford means- 
of eftimating and comparing the errors in the angle deduced, 
occafioned by the unavoidable errors in obfervation and practical 
conftruCtion ; an examination extremely ufeful in aftronomical 
fubjeCts : next to removing errors entirely from obfervation s* 
which is fcarcely to be hoped for, the leffening, circumfcribing, 
and reducing, them within known limits is an objeCt of prin- 
cipal confequence. 
20. The conftruCtion of fig. 2. remaining, through the 
points F and I (fig. 14.) draw the great circle FI. BifeCt FI in 
and through the points K and Q^draw the arc KQ^ which * 
■will be perpendicular to IF. To determine by computation the 
arc ED which meafures the angle fubtended by the obferved 
objeCts, three fpherical triangles, KQF or KIF, IFB, and 
DBE, muft be folved, for which the data are evidently fufficient ; 
5 or 
