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facilitate the underftanding of this method, it will be 
neceffary to premife the following obfervation. 
Obf. ii. Rays of light, which are brought to 
fuch convergency as to form the image of an objedt, 
proceed, after that, diverging, in the manner they did 
when they iftiied from the objedt before they were 
tranfmitted thro’ the glafs ; and therefore they may 
be again colledted by another fpherical glafs, fo as to 
form a fecond reprefentation of the fame objedt; 
which may again be repeated by a third glafs, &c. 
So that the firft image may be conf der’d as an objedt 
to the fecond glafs, and the fecond image will be an 
objedt to the third, and fo on. Tho’ thefe images 
may be very different, in refpedt to their magnitudes, 
yet they will be all fi milar ; being true reprefenta- 
tions of the fame objedt : This will hold good, tho’ 
the fecond glafs fhould be put fo near the firft as to 
receive the rays before the image is form’d: For as 
the rays are tending to meet at a certain didance, the 
fecond will receive them in that decree of conver- 
O 
gency, and, by an additional refradtion, bring them 
to a nearer focus ; but the image will dill be fimilar 
to that which would have been made by the firft glafs, 
if the fecond had not been there. 
Upon this principle all refradting telefcopes are 
made ; fome of which are a combination of four, 
five, or fix, glades. The firft glafs forms an image 
of the objedt ; the fecond repeats the image, which 
it receives from the firft ; and fo on, till the laft glafs 
brings a true reprefentation of the objedt to the eye. 
The fame may be faid of refledting telefcopes : For 
a. fpherical mirror adts in the fame manner, in that 
refpedt, as a fpherical glafs. 
Now 
