of the Angle fubiended by Two ObjeBs, &e. 41 1 
are wholly independent or the inclination o( the telefcope to the 
plane of motion, which is regulated to its true fituation parallel 
to the plane OP A, by making the plane OPA fixed in regard to 
diftant objects, and by observing if the images of objects E, feen 
after two refledtions of the rays, defcribe the arc of a great circle 
while C is carried round the plane of motion. Any three fixed ob- 
jects, at afufficient diftance, and fituated in the fame plane with 
the obferver’s eye, will be fufficient for making this adjuftment. 
Fig. 9. 10. and 11, reprefent the progrefs of the rays, and 
the pofition of the refiedtors according to this conftrudliom TC 
is a ray ifluing from any object T in a direction 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 
reflected in the diredtion CB perpendicular to the plane OCA, 
and being there refledted by the fpeculum B proceeds in the given 
or conftant diredtion EG parallel to the plane of motion OPA. 
16. There is another conftrudtion which follows from the 
general theory, the defcription of which fhould not be 
omitted. This will require fome little explanation. As be- 
fore (fig. 12.) let OPA reprefent the plane of motion, K its 
pole, FIM a parallel or lefler circle projedted on it, the diftance 
of this parallel from the pole K being meafured by the arc PK ; 
let KO be the primitive fecondary, and BFD the fixed plane of 
reflection on the fpeculum B coincident with it. The other 
parts of the conftrudtion * remaining, it has been demonftrated 
(art. 10 ), 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 DJL 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 decreafed, 
* Compare fig. 2. and 3. 
