Opticks 1717 - Up

And fince thet’e D$pofitions.- were in the Rays,
before their Incidence on the, fecond, third and
fourth Surfaces of the two Cryftals, and lilffercd
no alteration (fo far as appcarS) by the RefraAion
of the Rays in their pa&ge through
thofe Surfaces, and -the wraps were refrahed by
the fkne Laws in all the four Surfaces ;a it appears
that thofe Dlfpofitions vere in the Rays
originally, and f&jkr’d no alteration by the firit
Mefra&ion3 and that by means of thofe DXpofi-,
tions the Rapt IV~:Te refix&d at their Incidence
on the firl.t kku%ce of the firit CryBal, Some of
them r-$ter the 'uli~al, ad fame of them after the
ullui-ual mzllller 7 accordingly as their Sides of
un~xf~d Refkai-tion were then turn’d towards
the Coait of the unufual RefraAion of that Cryitil,
or iideways from it.
Evesy Ray of Light has therefore two oppofite
Sides, originally endued with a Property
on which the unufual RefiakIion depends, and
the other 0170 oppofite Sides not endued with
that Property. And it remains to be enquired,
whether there are not more Properties ofLight
by which the Sides of the Rays difI%x, and are‘
diltinguifll’d from one another.
In explainin, m the dikkrence of the Side’s of.
the Rays above mention’d, E have fkppofed that
the Rays fall perpcndicula:ly on the firfi Cry-,
Ital. But if they fall obliquely on it ,. tlie.§uccefs
is the hne. Thoi‘e Rays wliizh are refra&-
ed ~fier tIif2 ~tiiial n~anncf in the firit: Cryfiaf;
Will be refr&ed after the unufual manner in
the fccond e’ryfial, iilppofing thePlanes ofperpendicular
~R&ra&.iora~ to be’at right hgles with
i-2 one