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# Title: Opticks: Or, A Treatise Of The Reflexions, Refractions, Inflexions and Colours Of Light (shelfmark Leibn. Marg. 193)
# Folio 7, page id 1677894601:0017
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same manner may che Retraction be Found when the
Lens is Conrex on one side and Plane or Concare on
che ocher, or Concare on boch Sides.
A X. VI.
Fhomogeneal Rays nhich Rom from severat Points of any Ob-
sect, and faii almost Perpendiculariy on any reftectine or refra-
etinz Plane or Spherical Surface, Pall aftermards diverge from
lo many ether Points, or be Parallel to so many other Lines, or
converge to se many other Points, either accurately or without any
sensible Frror. And the same thing will happen, if the Rays be
reflected or refracted successively by tuo or three or more Plane
or spherical Surfaces.
The Poine Erom Which Rays diverge or eo which chey
conrerge may be called cheir Focus. And the FocuS ot
che incident Raysbeing giren, thac ot che reflected or re-
Fracted ones may be tound by finding the Retraction ot
any trvo RaYS, as above; or more readily chus.
Cas. 1. Let ACB BC 2 retlectinq or retracting Plane, Fiz. 4.
and 2 che Focus oF che incident Rays, and C q C 2 per-
pendicular eo thac Plane. And il this perpendicular be
produced eo q (0 thac qC bc equal eo C, the point q
Thall be the Focus ot the refected Rays. Or it gC bc
cakcen on che Pame sde et che Plane wrich C and in PrO-
portion co C as che sine ot Incidence co the sine ot
Retraction, che point q Phall be the Focus ot che retrac
ted Rays.
Cas. 2. Tet A C B Be che rehecrins SurFaCC OF anY Fig. 5.
Sphere whole Center is F. Bileet any Radius thereot (sup-
Dosc F C) in Y, and if in chac Radius on the lame Side che
Poine Prou cake che Poines  and q. (0 that I O, TF,
aud Tg be continual Proportionals, and che point Q be
tlac