of the Angle fuht ended by Two Objehls, &c. 397 
on the fubjed, and fha'll 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- 
fledions unconfined to any particular cafe may be defcribed 
thus. Let C, B (fig. 1.) reprefent two plane receding furfaces, 
inclined to a plane OPA at any given angle. Through any 
point of the refleding furface C draw a line perpendicular to 
the plane OPA, and with the point where the line meets the 
plane as a centre (which mud here be reprefented by C) and 
any didance CP, defcribe a circle OPA. The refleding plane B 
always continuing fixed, let the refledor 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 refiedors, B and C, being equally in- 
clined to the plane OPA, it follows, that during the motion of 
C there mud be fome point O in the circumference OAP, at 
which when CP arrives, the refledor C will be parallel to the 
fixed refledor B. 
3. When the moveable radius which carries round the plane 
C is at aiiy other pofition CP 9 let a ray flowing from a didant 
objed T impinge on the fpeculum C ; let it be refleded from 
thence in the diredion CB, and being again refleded at B in the 
diredion BG, let it be obferved by a fpeitator’s eye at G ; the 
image of T will appear fomewhere in the line GBS ; fuppofe 
that a ray flows from a didant objed S fituated in the line GB 
produced, and that this ray SG comes diredly to the fpedator’s 
Vo l. LXXI. G g g eye 
1 
