Oil of turpentine and quartz
are examples; but Faraday showed that, while in one direction,
that is, across the lines of magnetic force, his rotation is zero,
augmenting gradually from this until it attains its maximum, when
the direction of the ray is parallel to the lines of force; in the
oil of turpentine the rotation is independent of the direction of
the ray. But he showed that a still more profound distinction
exists between the magnetic rotation and the natural one. I will
try to explain how. Suppose a tube with glass ends containing oil
of turpentine to be placed north and south. Fixing the eye at the
south end of the tube, let a polarized beam be sent through it from
the north. To the observer in this position the rotation of the
plane of polarization, by the turpentine, is right-handed. Let the
eye be placed at the north end of the tube, and a beam be sent
through it from the south; the rotation is still right-handed.
Not so, however, when a bar of heavy glass is subjected to the
action of an electric current. In this case if, in the first
position of the eye, the rotation be right-handed, in the second
position it is left-handed. These considerations make it manifest
that if a polarized beam, after having passed through the oil of
turpentine in its natural state, could by any means be reflected
back through the liquid, the rotation impressed upon the direct beam
would be exactly neutralized by that impressed upon the reflected
one.
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