of the Angle fubt ended by dl’wo Objects, &c. 4 1 3 
fhould next be defcribed ; thefe will be more obvious if an out- 
line be annexed, reprefenting this particular cafe of the theory 
adapted to the menfuration 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 objedt T im- 
pinges on the fpeculum C, and being reflected in the direction 
CB, is there again refledted in the direction BG, patting along 
the axis of a telefcope. A ray coming from another diftant 
objedt S, inclined to the ray TC at a fmall angle enters the tele- 
fcope parallel to the diredtion of its axis, which is coincident with 
1 BG, and confequently the images of the two objedts 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. 
1 7 - By this figure (fig. 13.), without further argument, it 
is plain, that the magnitude of the reflebting fpeculum C limits 
the conflant 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 refledling plane de- 
pends on the quantity of light required ; if a circle of about 
1 2 inches diameter be lufficient, and the perpendicular diftance 
of the refledting planes be made equal to five inches, the leaf! 
angle of incidence, confident with thefe conditions, will be 
about 7 0 . It is however to be remembered, that the magnitude 
of the refledlors fhould be adapted to the aperture of the tele- 
fcope ufed 111 the obfcrvation. As the area of the fpeculum in— 
cieafes, the light admitted into the fame telefcope decreafcs, 
and thefe areas fhould be fo proportioned as to afford equal quan- 
tities of light, fo that the objedfs feen by two refedtions, and 
by diredt rays, may be nearly of equal brightnefs ; but for the 
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