perpendicular Refraction. And the Coast towards which the lines KL and
VX are drawn, may be call'd the Coast of unusual Refraction.
In like manner Crystal of the Rock has a double Refraction: But the
difference of the two Refractions is not so great and manifest as in
Island Crystal.
When the beam ST incident on Island Crystal is divided into two beams TV
and TX, and these two beams arrive at the farther Surface of the Glass;
the beam TV, which was refracted at the first Surface after the usual
manner, shall be again refracted entirely after the usual manner at the
second Surface; and the beam TX, which was refracted after the unusual
manner in the first Surface, shall be again refracted entirely after the
unusual manner in the second Surface; so that both these beams shall
emerge out of the second Surface in lines parallel to the first incident
beam ST.
And if two pieces of Island Crystal be placed one after another, in such
manner that all the Surfaces of the latter be parallel to all the
corresponding Surfaces of the former: The Rays which are refracted after
the usual manner in the first Surface of the first Crystal, shall be
refracted after the usual manner in all the following Surfaces; and the
Rays which are refracted after the unusual manner in the first Surface,
shall be refracted after the unusual manner in all the following
Surfaces. And the same thing happens, though the Surfaces of the
Crystals be any ways inclined to one another, provided that their Planes
of perpendicular Refraction be parallel to one another.
And therefore there is an original difference in the Rays of Light, by
means of which some Rays are in this Experiment constantly refracted
after the usual manner, and others constantly after the unusual manner:
For if the difference be not original, but arises from new Modifications
impress'd on the Rays at their first Refraction, it would be alter'd by
new Modifications in the three following Refractions; whereas it suffers
no alteration, but is constant, and has the same effect upon the Rays in
all the Refractions. The unusual Refraction is therefore perform'd by an
original property of the Rays. And it remains to be enquired, whether
the Rays have not more original Properties than are yet discover'd.
_Qu._ 26. Have not the Rays of Light several sides, endued with several
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