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# Title: Opticks: Or, A Treatise Of The Reflexions, Refractions, Inflexions and Colours Of Light (shelfmark Leibn. Marg. 193)
# Folio 8, page id 1677894601:0018
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he FOcus oF che incidene Rays, che poine q Pall bc chc
Focus oF che rellected ones.
Cas. 3. Let ACB bC the retractin Surtace ot any
Sphere nhose Center is F. In any Radius chereof L C
produced boch mars rake F. T and Ct Sererally in Cuch
Proportion co that Radius as the leller ot che sines ot
Incidence and Retraction hach co che difference et those
Sines. And chen il in che Came Line ron And any evo
Points  and g, (0 chac T ( bC EOF T aS F FEO t9.
cakcins t q the contrary may trom tirhich T C liech From
l, and it che Poine be the Focus ot any incidene RaYS,
the Point g Mall be che Focus ot che retracted ones.
And by che Came means che Focus of che Rars atrer
ervo or more Reherions or Retractions mar be Found.
Cas. 4. L et A C BD bc any retractins Lens, spheri-
cally Conrex or Concare or Plane on cicher side, and let
CD bC ies Aris (thac is the Line irhich curs boch ies Sur-
faces perpendicularir, and passes chrouoh che Centers ot
che Spheres,) and in this Aris lec F and fbc the Foci ot che
retracted Rays Found as abore, irhen che incident Rays
on boch Gidesthe Lens are Parallel cothe lame Aris; and
uxon the Diamerer F f bilecced in F, describe a Circle.
Suppole non chat anx Poine Q bc che Locus ot any inci-
dene RayS. Dra LP curting che laid Circle in P andt
and cherein cake t. g in such Proportion to + F aStF Or TF
hathco TO. Lec tg lye che contrary way trom tirhich
TO doih trom T, and g Thall. be the Focus of the retrac
ced Rays ithout aur sensible Error, provided che Poinr
Obc noc lo remore trom che Axis, norche Lens (o brond
25 eo makie any oF che Rars FaII too obliquely on che
retractino Surtaces.
And by che like Operations mar che reflecting or re-
traccins Surtaces be tound nhen cherro Foci are giren,
and
. 6