/ 
of the Angle fait ended by Two Objedls^ &c. 397 
on the fubjeCt, and fhall be happy if the refult of my in- 
quiries appears to merit the attention of the Royal Society. 
Art. 1. The manner of taking an obfervation by two re- 
flections unconfined to any particular cafe may be defcribed 
thus. Let C, B (fig. 1.) reprefent two plane reflecting furfaces, 
inclined to a plane OPA at any given angle. Through any 
point of the reflecting furface C draw a line perpendicular to 
the plane OPA, and with the point where the line meets the 
plane as a centre (which mull here be reprefented by C) and 
any diftance CP, defcribe a circle OPA. The reflecting plane B 
always continuing fixed, let the refleCtor C be moveable along 
with the radius CP as it revolves in the plane OPA round the 
centre C : the angular motion of the fpeculum C, referred 
to the circumference OPA, will be meafured by the arc which 
the radius CP defcribes, the inclination of the plane C to the 
plane of motion OPA being always the fame, and equal to that 
of the fixed fpeculum B. 
, 2. The two plane refleCtors, B and C, being equally in- 
clined to the plane OPA, it follows, that during the motion of 
C there muft be feme point O in the circumference OAP, at 
which when CP arrives, the refleCtor C will be parallel to the 
fixed refleCtor B. 
3. When the moveable radius which carries round the plane 
C is at any other pofition CP, let a ray flowing from a diftant 
objeCt T impinge on the fpeculum C ; let it be reflected from 
thence in the direction CB, and being again reflected at B in the 
direction BG, let it be obferved by a fpeCtator’s eye at G ; the 
image of T will appear fomewhere in the line GBS; fuppofe 
that a ray flows from a diftant objeCt S fituated in the line GB 
produced, and that this ray SG comes direCtly to the fpectator’s 
Vol, LXXI. G g g eye 
