3^8 Mr. atwood* s Theory for the Menfuration 
eye at G : the objeCt S feen by direct rays, and the image of 
the point T feen by rays after two reflections, will appear to 
coincide in the line GBS. This is an obfervation by two 
reflections, from which, together with fuch data as limit the 
problem, the true angle fu blended by the objects T and S is to 
be inferred* 
4 # The data which limit this problem, being neceffary fot 
the determination of the angle fubtended by T and S are in 
number four, which are next to beconfidered. ift. One of thefe 
data is the arc PO, being the angular diftance of the moveable 
radius CP, meafured on the circumference of the circle OPA* 
from that petition CO, at which the two refleCtors are parallel ; 
the fituation of this arc OP in refpeCt of the point O being 
fuppofed known, that is, it being known on which fide of that 
point, OP is fituated in refpeCt of the ray BG. 2dly, The com- 
mon inclination of the reflecting planes B and C to the plane of 
motion is another of thefe data. The third and fourth of 
the conditions muft be mentioned rather more particularly* 
The ray BG is always underftood to be given in pofition in 
refpeCt of the plane of motion OP A (confidered as immoveable) 
being either coincident with the line of collimation of a tele- 
fcope, or direfted 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 reflection. 
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 conftant angle of in- 
cidence or reflection at the fixed fpeculum B will be 
another of the data neceffary to determine the problem. 
The 
