804 Dr. maskelyne’s Account of 
from the prifm and fall upon the objedt-glafs parallel to 
the rays ee, and confequently will have their focus zw 
coincident with the focus e of the rays ee, and therefore 
the two images of the Sun we, zzvze, will touch one 
another externally at the point ez w, and the inftrument 
will meafure the angle EC2W, and that only. 
But if the prifm be placed within the telefcope, the 
angle meafured by the inftrument will be to the refrac- 
tion of the prifm as the diftance of the prifm from the 
focus of the objedt-glafs is to the focal diftance of the 
objedt-glafs : or if two prifms be ul'ed to form the two 
images, with their refradting angles placed contrary 
ways, as reprefented in fig. 3. and 4. the angle meafured 
will be to the fum of the refractions of the prifms as the 
diftance of the prifms from the focus of the objedt-glafs 
is to the focal diftance of the objedt-glafs. For let acb 
(fig. 3.) reprefent the objedt-glafs, and d the eye-glafs of 
a telefcope, and pr, rs, two prifms interpofed between 
them, with their refradting angles turned contrary ways, 
and the common fedtions of their refradting planes 
touching one another at r. The rays proceeding from 
an objedt, fuppofe the Sun, will be difpofed, by the re- 
fradtion of the objedt-glafs, to form an image of the Sun 
at the focus; but part of them falling on one prifm, and 
part on the other, will be thereby refradted contrary 
ways, fo as to form two equal images we, zwze, which, 
if 
