of the Angle fuhtended by Two Objeffis, &c. 399 
The rays GB, BC, and the fpeculum B, being fixed in refpedt 
of each other, and of the plane OAP, the plane CBG will 
alfo be given in pofition ; that is, its inclination to the plane of , 
motion, or to any other fixed plane, will conftantly be the 
fame : whereas the inclination of the plane BCT to the plane 
of motion, or other fixed plane, will be continually changing 
while the refledtor C revolves with the radius CP. The pofition 
of the plane GBC conftiiutes the fourth and laft of the data? 
and it will be immaterial to what fixed plane it is referred. In 
the enfiiing folution the fituation of this plane will be defined 
by its inclination to the fixed fecondary of the plane of motion 
which paffes through the pioint O. 
5. The enumeration of thefe data leads to the conftruc- 
tion of the problem, a few obfervations being previoufly 
inferted to prevent repetitions and unneceflary references, ift, 
The objedts obferved are underftood to be lucid or illumined 
points, and fo diftant, that the rays which flow from either of 
them may be efteemed parallel without error as far regards 
thefe obfervations : fuch objedts are the fixed ftars, any given 
points in the difks of the fun or planets, &c, 2dly, As in mea- 
furing the angular pofition s of objedts which lie in the fame 
plane, thefe objedts are referred to the circumference of a circle, 
the centre of which is coincident with the fpedtator’s eye ; fo 
in eftimating the positions of objedts which lie in different planes, 
and of the inclinations of thefe planes to each other, the ob- 
jedts, &c. are referred to the circumference of a fphere, of 
which the centre coincides with the centre of the fpedtator’s 
view : applying this to the prefent cafe, fince the lines 
CT, CB, SG (fig. 1 .) are fituated in different planes ; in order 
to eftimate their pofitions, any point may be aflumed as 
the centre of a fphere, and through that point lines are to be 
drawn parallel to the given lines CT, CB, SG, the points in 
G g g 2 which 
