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PRECIOUS STONES.
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cylindrical
lens the line of separation between light and dark appears as a
straight line. The range is from 1.40 to 1.76 in value of n.
2.
Double refraction. All crystals except those belonging to the cubic
system show double refraction ; that is to say, a ray of monochromatic
light passing through such a crystal is not only deviated from its
direction without the crystal, but is turned in two directions, part of
it going in one path and part in another; the angle between these two
paths is very small, never more than a few degrees at most, and in some
cases is so small that the two rays would seem to be one except when
determined by special apparatus.
This
double refraction can be readily shown if we take a piece of Calcite of
the clear variety known as Iceland Spar or Doubly Refracting Spar, of
the form it naturally assumes when broken, and view through it a mark,
such as a cross, on a piece of paper. Two crosses appear, both somewhat
removed from the position we see the original mark in, when nothing is
interposed, but one more displaced than the other. On rotating the
Iceland Spar, one cross appears to move round the other. Similarly, if
we view an object through a cut specimen of one of the doubly
refracting gems such as Peridot, we shall see two images of the object
through each of the facets on the part of the gem away from the eye. On
rotating a piece of Iceland Spar as above, we should also notice that
in certain positions the two images were further apart than in others;
now if the mineral were cut in a particular direction and the surfaces
polished, on looking at the cross through it we should only see one
image; in other words, it is singly refracting in that direction. Such
directions are known as optic axes;
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