[ 55 6 1 
points C D the centres of the two glafs 
fegments : Alio G the focus where the 
images of the extremities of the object 
are coincident. It is evident, from 
Obf. that AG and BG are ftrait 
lines, that pafs thro’ the centres of the 
fegments, and conned: the extreme 
points of the objed with their cor- 
refponding points in the images ; and 
therefore, as the diameter of the objed 
and the diftance between the centres of 
the fegments are both infcribed be- 
tween thefe two lines, they mud: needs 
fubtend the fame angle from the point 
where thofe lines meet ; which is at G. 
^ E. D. 
The focal didance C G, or D G, is 
variable, according to the diftance of 
the objed from the glafs : So that it 
decreases as the diftance of the objed 
from the glafs increnfes ; and when the 
objed is fo far off, that the focal length 
of the glafs bears no proportion to its 
diftance; then will it be leaft of all, 
as C F or D F -, and the point F is 
call’d the focus of parallel rays. Any 
other focus, as G, being the focus 
of a near objed, is call’d a refpedive 
focus ; as it refpeds a particular di- 
ftance : But the focus of parallel rays 
refpeds all objeds that are at a very 
great diftance; fuch as is that of all the 
heavenly bodies. 
P a o- 
