of the Angle fublended by Two Objects, See. 41 1 
are wholly independent of the inclination of the telefcope to the 
plane of motion, which is regulated to its true fituation parallel 
to the plane OPA, by making the plane OPA fixed in regard to 
diffant objects, and by obferving if the images of objedts E, feen 
after two refledtions of the rays, deferibe the arc of a great circle 
while C is carried round the plane of motion. Any three fixed ob- 
jects, at a fufficient difiance, and fituated in the fame plane with 
the obferver’s eye, will be fufficient for making this adjuflment. 
Fig. 9. 10. and ix. reprefent the progrefs of the rays, and 
the pofition of the refledlors according to this confiru diion. TC 
is a ray Bluing from any objedt T in a diredtion parallel to the 
plane of the motion, and impinging on the fpeculum C, which, 
is inclined to that plane at an angle of 45 0 : from hence it is 
.refledted in the diredtion CB perpendicular to the plane OCA, . 
and being there -refledted by the fpeculum B proceeds in the given 
or conftant diredtion BG parallel to the plane of motion OPA. 
x 6 . There is another conftrudtion which follows from the 
general theory, the defeription of which fhould not be,- 
omitted. This wdll require fome little explanation. As be- 
fore (fig. 12.) let OPA. reprefent the plane of motion, K its 
pole, F 1 M a parallel or leffer circle projedted on it, the difiance 
of this parallel from the pole K being meafured by the arc FK ; 
let KO be the piimitive fecondary, and BFD the fixed plane of 
refiedtion on the fpeculum B coincident with it. The other 
parts of the conftrudtioii * remaining* it has been demon firated 
(art. 30. ), that the fine of half the obferved angle, that is, 
the fine of half ED, is proportional to the fine of IF, and 
the cofine of DL jointly. Every thing elfe being the fame, 
it is manifeft, that the fine of half ED will be proportional to 
the fine of IF : as therefore the arc KF, that is, the inclina- 
tion of the refledting planes to the plane of motion, is decreafecl, 
* Compare fig. 2, and 3. 
