a new prifmatic Micrometer. 805 
if the refractions of the prifms be of proper quantities, 
will touch one another externally at the point eiw. Let 
ecn be the axis of the pencil of rays ee proceeding from 
the Sun’s Eaftern limb; and wco the axis of the pencil 
of rays ww proceeding from the Sun’s Weftern limb ; 
and the point n the place where the image of the 
Sun’s Eaftern limb would be formed, and the point o 
where that of the Weftern limb would be formed, 
were not the rays diverted from their courfe by the 
refractions of the prifms. But by this means part 
of the rays ee, which were proceeding to n, falling 
on the prifm PR, will be refracted to form an image of 
the Sun’s Eaftern limb at e , while others of the rays ee, 
which fall on the prifm rs, will be refraCted to form an 
image of the Sun’s Eaftern limb at ie. In like manner, 
part of the rays ww, which were proceeding to form an 
image of the Sun’s Weftern limb at o, falling on the 
prifm rs, will be refracted to form an image of the Sun’s 
Weftern limb at iw coincident with e, the point of the 
image correfpondent to the Sun’s Eaftern limb; while 
others of the rays ww, which fall on the prifm pr, will be 
refraCted to form the image of the Sun’s Weftern limb at 
w. The two images we, zwxe, are fuppofed to touch one- 
another externally at the point er w. The ray efr, which 
belongs to the axis ecn, and is refraCted by the prifm 
er to e, undergoes the refraCtion nr<?, which (be- 
caufe 
