414 Mr. atwood’s Theory for the Menfurat'wn 
fake of conflrudting an example to this theory, the magnitude 
of the refledling planes and the angle of incidence on the fixed 
fpeculum B depending on it may be affirmed of the value men- 
tioned in this article. 
1 8.. The magnitude of the arc KF, or of the inclination of 
the reflebfing planes to the plane of motion is limited by the 
angles which the obferved objedts fubtend (art. i6.)y Fig. 12. 
Becaufe the greatefF angle obfervable wdl be ineafured by four 
times the arc KF, it follows, that the are KF muff not be lefs- 
than one fourth part of the greateft angle intended to be ob- 
ferved by this conflrudion ; 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 conflrudiion (fig. 
2), the adtual meafure of the angle fubtended by the objedls* 
obferved. This muff be effedled by computation, which will: 
not only ferve as an illuflration of the theory, but afford means- 
of eflimating and comparing the errors in the angle deduced,, 
occafioned by the unavoidable errors in obfervation and practical 
conflrudfion ; an examination extremely ufeful in aflronomical 
fubjedts : next to removing errors entirely from obfervation s, ; 
which is fcarcely to be hoped for, the leflening, circumfcribing, 
and reducing them within known limits is an objedl of prin- 
cipal confequence. 
20. The conftrudtion of fig. 2. remaining, through the 
points F and I (fig. 14.) draw the great circle FI. Bifedt 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 
, objedts, three fpherical triangles, KQF or KIF, 1 FB, and 
DBE, mull be folved, for which the data are evidently fufficient ; 
5 
or 
