takes a Polish very difficultly. It polishes better upon polish'd
Looking-glass than upon Metal, and perhaps better upon Pitch, Leather or
Parchment. Afterwards it must be rubb'd with a little Oil or white of an
Egg, to fill up its Scratches; whereby it will become very transparent
and polite. But for several Experiments, it is not necessary to polish
it. If a piece of this crystalline Stone be laid upon a Book, every
Letter of the Book seen through it will appear double, by means of a
double Refraction. And if any beam of Light falls either
perpendicularly, or in any oblique Angle upon any Surface of this
Crystal, it becomes divided into two beams by means of the same double
Refraction. Which beams are of the same Colour with the incident beam of
Light, and seem equal to one another in the quantity of their Light, or
very nearly equal. One of these Refractions is perform'd by the usual
Rule of Opticks, the Sine of Incidence out of Air into this Crystal
being to the Sine of Refraction, as five to three. The other
Refraction, which may be called the unusual Refraction, is perform'd by
the following Rule.
[Illustration: FIG. 4.]
Let ADBC represent the refracting Surface of the Crystal, C the biggest
solid Angle at that Surface, GEHF the opposite Surface, and CK a
perpendicular on that Surface. This perpendicular makes with the edge of
the Crystal CF, an Angle of 19 Degr. 3'. Join KF, and in it take KL, so
that the Angle KCL be 6 Degr. 40'. and the Angle LCF 12 Degr. 23'. And
if ST represent any beam of Light incident at T in any Angle upon the
refracting Surface ADBC, let TV be the refracted beam determin'd by the
given Portion of the Sines 5 to 3, according to the usual Rule of
Opticks. Draw VX parallel and equal to KL. Draw it the same way from V
in which L lieth from K; and joining TX, this line TX shall be the other
refracted beam carried from T to X, by the unusual Refraction.
If therefore the incident beam ST be perpendicular to the refracting
Surface, the two beams TV and TX, into which it shall become divided,
shall be parallel to the lines CK and CL; one of those beams going
through the Crystal perpendicularly, as it ought to do by the usual Laws
of Opticks, and the other TX by an unusual Refraction diverging from the
perpendicular, and making with it an Angle VTX of about 6-2/3 Degrees,
as is found by Experience. And hence, the Plane VTX, and such like
Planes which are parallel to the Plane CFK, may be called the Planes of
203