2 Vfr. Atwood’s Theory for the Menfuratlon 
eye at G : the object S feen by direct rays, and the image of 
the point T feen by rays after two reflexions, will appear to 
coincide in the line GBS. / This is an obfervation by two 
refleXions, from which, together with fuch data as limit the 
problem, the true angle fubtended by the objeXs T and S is to 
be inferred. 
4. The data which limit this problem, being neceflary for 
the determination of the angle fubtended by T and S are in 
number four, which are next to be confidered. 1 A. One of thefe 
data is the arc PO, being the angular difiance of the moveable 
radius CP, meafured on the circumference of the circle QPA, 
from that pofition CO, at which the two refleXors are parallel ; 
the Atuation of this arc OP in refpeX of the point O being 
fuppofed known, that is, it being known on which fide of that 
point, OP is f tuated in refpeX of the ray BG. 2dly, The com- 
mon inclination of the refleXing planes B and C to the plane of 
motion is another. of thefe data. The third and fourth of 
the conditions muA be mentioned rather more particularly. 
The ray BG is always underAood to be given in pofition in, 
refpeX of the plane of motion OPA (confidered as immoveable) 
being either coincident with the line of collimation of a tele- 
fcope, or direXed by fights fo as to be invariably fixed : the fpe- 
culum B alfo being unmoved, the line or ray BC will never 
change its pofition, from the known principles of refleXion. 
The angle CBG, therefore, and the half of that angle 
being the angle of incidence at which CB. impinges on B, 
will be always of the fame magnitude ; whereas the half of 
the angle BCT, or the angle of incidence on the moveable 
fpeculum C, is continually changing, while C is carried 
round in the plane of motion: this eonfiant angle of in- 
cidence or refleXion at the fixed fpeculum B will be 
another of the data neceflary to determine the problem. 
The 
