of the Angle fubt ended by "Two OhjeAs^ &c. 4 1 3 
fliould next be defcribed ; thefe will be more obvious if an out- 
line be annexed, reprefentmg this particular cafe of the theory 
adapted to the menfu ration of fmall angles when reduced to 
practice. OAPC (fig. 13.) is the plane of motion, C the 
moveable fpeculum carried round in the plane of motion by 
the radius CP ; a ray coming from any diftant objeCt T im- 
pinges on the fpeculum C, and being reflected in the direction 
CB, is there again reflected in the direction BG, palling along 
the axis of a telefcope, A ray coming from another diftant 
object S, inclined to the ray TO at a fmall angle enters the tele- 
fcope parallel to the direction of its axis, which is coincident with 
EG, and corifequently the images of the two objects S and T will 
be feen to coincide in the middle of the field of the telefcope, 
the angle fubtended by them being BGT, which muft be deter- 
mined by the fubfequent computations. 
17. By this figure (tig. 13.), without further argument, it 
is plain, that the magnitude of the reflecting fpeculum C limits 
the conftant angle of incidence on B ; for were that angle = 0 , the 
lines CB, BG, would coincide, by which means the ray BG 
and others adjacent to it would be intercepted from enter- 
ing the telefcope. The magnitude of the reflecting plane de* 
pends on the quantity of light required ; if a circle of about 
I 2 inches diameter be lufficient, and the perpendicular diftance 
of the reflecting planes be made equal to five inches, the leaft 
tingle of incidence, confident with thefe conditions, will be 
about 7 0 . It is however to be remembered, that the magnitude 
of the reflectors fhould be adapted, to the aperture of the tele^ 
fcope ufed in the obfervatioh. As the area 4 of the fpeculum in- 
creafes, the light admitted into the fame . telefcope decreafes, 
and thefe areas fhould be lb proportioned as to afford equal quan- 
tities of light, fo that the objects feen by two reflections, and 
by direCt rays, may be nearly of equal brightncfs ; but for the 
Vol. LXXI. I i i lake 
