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STRUCTURE AND LIGHT OF THE SKY. 255

the case of an ordinary beam of light, the vibrations of the
ether-particles are executed in every direction perpendicular
to it; but let the beam impinge obliquely upon a plane-
glass surface, as in the case of Malus, the portion reflected
Will no longer have its particles vibrating in all directions
round it. By the act of reflection, 2:)“ it occur at the proper
angle, the vibrations are all confined to a single plane, and
light thus circumstanced is called plane polarized light.

A beam of light passing through ordinary glass executes
its vibrations within the substance exactly as it would do
in air, or in ether-filled space. Not so when it passes
through many transparent crystals. For these have also
their two-sidedness, the arrangement of their molecules
being such as to tolerate Vibrations only in certain definite
directions. There is the Well-known crystal tourmaline,
Which shows a marked hostility to all Vibrations executed
at right angles to the axis of the crystal. It speedily ex-
tinguishes such vibrations, while those executed parallel to
the axis are freely propagated. The consequence is, that a
beam of light, after it has passed through any thickness of
this crystal, emerges from it polarized. So also as regards
the beautiful crystal known as Iceland spar, or as double-
refracting spar. In one direction, but in one only, it acts
like a piece of glass; in all other directions it Splits the
beam of light passing through it into two distinct halves,
both of which are perfectly polarized, their vibrations being
executed in two planes, at right angles to each other.

It is possible by a suitable contrivance to get rid of one
of the two polarized beams into which Iceland spardivides
an ordinary beam of light. This was done so ingeniously
and effectively by a man named Nicol, that the Iceland spar,
cut in his fashion, is now universally known as N icol’s prism.
Such a'prism can polarize a beam of light, and if the beam,
before it impinges on the prism, be already polarized, in
one position of the prism it is stopped, while in another

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