r 552 ] 
point of a luminous objedt, to fome other point; 
whofc diftance from the glafs depends chiefly on its 
convexity, and the diftance of the objedt from it. 
Obf 2. The point, where the rays are thus col- 
ledted, may be confldered as the image of that point, 
from which they diverge. For if we conceive feveral 
radiant points thus emitting rays, which, by the re- 
fradtive quality of the glafs, are made to converge to 
as many other points ; it will be an eafy matter to 
underftand, how every part of the objedt will be truly 
reprefented. As this property of fpherical glaffes is 
explained and demonftrated by all the writers on op- 
tics ; it being the very foundation of the fcience, the 
bare mention of it is fufticient for the prefent purpofe. 
Obf. 3. It will be neceffary, however, to obferve 
farther, that the lines connedting every point in the 
objedt with its correfponding ones in the image, do 
all interfedt in a certain point of the axis or line pair- 
ing thro’ the poles of the glafs, where its two furfaces 
are parallel, and may be properly called its centre : 
Whence it appears, that the angles fubtended by the 
objedt and its image from that point, muft be equal : 
And therefore their diameters will be in the fame 
ratio, as their diftances from that point. 
Obf. 4. As the formation of the image by the glafs 
depends entirely on the property above-mentioned, 
that is to fay, its colledting all the light, that is in- 
cident on it, from the feveral points of the objedt into 
as many other points at its focus ; it follows, that any 
fegmcntof fuch a glafs will alfo form an image equal, 
and every way fimilar, to that exhibited by the whole 
glafs ; with this difference only, that it will be fo 
much darker, as the area of the fegment is lefs than 
that of the whole slafs. 
Obf 
