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ments in any direction whatever; yet the mod conve- 
nient way for this purpofe is, to Aide their drait edges 
one along the other, till they are re- 
mov’d, as the figure in the margin re- 
prefents them : For thus they may be 
moved without differing any falfe light 
to come in between them. And by this 
way of removing them, the didance be- 
tween their centres may be very conve- 
niently meafured ; viz. by having a Vernier’s divi- 
fion, commonly, tho’ falfely, call’d a Nonnius’s, fixed 
to the brafs-work, that holds one fegment, fo as to 
Hide along a fcale on the plate, to which the other 
part of the glafs is fitted. 
Obf. 9. As the images of the fame objedt are fe- 
parated, by the motion of the fegments ; fo thofe of 
different objedts, or different parts of the fame objedt, 
may be made to coincide, Suppofe the fun, moon, 
or any planet, to be the objedt; the two images 
thereof may, by this contrivance, be removed, till 
their oppofite edges are in contadf : In which cafe, 
the didance between the centres of the two images 
will be equal to the diameter of either ; and fo of any 
other object whatever. 
Obf. 10. This divided glafs may be ufed, as a mi- 
crometer, three different ways. In the fird place, it 
may be fixed at the end of a tube, of a fuitable length 
to its focal didance, as an objedt-glafs ; the other end 
of the tube having an eye-glafs fitted as ufual in adro- 
nomical telefcopes. Secondly, It may be applied to 
the end of a tube mucli fhorter than its focal didance, 
by having another convex glafs within the tube, to 
fhorten the focal didance of that, which is cut in two. 
Ladiy, It may be applied to the open end of a re- 
fledting 
