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Obf. y. The axis of a fpherical glafs is a line con- 
iiefting the centres of the fpheres, to which the two 
furfaces are ground ; and where-ever this line palfes 
thro’ the glafs, there the furfaces are parallel. But if 
it happens, that this line does not go thro’ the fub- 
ftance of the glafs, fuch a glafs is faid to have no in- 
ternal centre; but it is conceived to be in its plane 
produced, till it meets the axis : And this imaginary 
point, tho’ external to the glafs, is as truly its centre, 
and is as fixed in its pofition to it, as if it were 
actually within its fubftance. 
ObJ\ 6. If a lpherical glafs, having its center or 
pole near its middle or centre of its circumference, 
fliould be divided by a ftrait line through the middle; 
the centre will be in one of the fegments only. For 
how exadt foever a perfon may be fuppofed to be in 
cutting it thro’ the centre ; yet tis hard to conceive, 
how a mathematical point fliould be divided in two : 
Therefore the centre will be internal to one of the feg- 
ments, and external to the other. But if a fmall mat- 
ter be ground away from the ftrait edge of each feg- 
ment, both their centres will become external ; and lo 
they will more eafily be brought to a coincidence. 
Ob/. 7. If thefe two fegments fliould be held to- 
gether, fo as to make their centres coincide ; the 
images, which they give of any objedt, will likewife 
coincide, and become a Angle one. This will be the 
cafe, when their ftrait edges are joined to make the 
glafs, as it were, whole again : But let the centres be 
any- how feparated, their images will aifo feparate, and 
each fegment give a feparate and diftindt image of any 
object, to which they may be expos’d. 
Obf. 8 . Tho’ the centres of the fegments may be 
drawn from their coincidence, by removing the ieg- 
4 . A ments 
