Opticks 1717 - Up

of thc&1ryM, C
foolid k’hgk at that
Surficc, G E Id F the oppofite Surface, and Q=K
a perpendkular on that Surface. This perpendicular
snakes with the edge of the Gryltal CF3
an Angle of 19 Ikgr. 3’. Join K F, and in it
take 16 L ) G.3 that the Angl
&. and the A.nglc L@F 12
ST reprcknc any beam of I_,
in any 1@glc upon the ref?aCting Surfixce J.A.,DBC;
let 1‘V be the refra&ed beratn determin’d by
the givenProportion of the Sines 5 to 3, actording
to the ul’ual Rule of Opticks. Draw VX
parallel and equal to IC L. Draw ir the fame
day .from v’ in which L lieth Corn K; and
joining T X, this line TX fhall be the other re:
fra14tcd. lmm carkxJ. from ‘IT to X, by the unl.lfllLII.
.Rdl~i~AiOll.
If therefore the incident beam ST be perpasdicular
to the refia&ing Surfixe , the two
beams ‘I’ V and T X3 into which it fhall become
divided, fl~all be parallel to the lines C K
and 4: I-1; one of thofe beams going through
the CrylM perpendicularly, as it ought to do
by the uktal Laws of Opticks, and the other
TX by an ui~ufual RcfraAion diverging from
the perpcndicalar 3 and mnking wirh it anAngle
vr;rx of ’‘I
q mut A