fully and anxiously,' but could not discover any
trace of action on the polarized ray.

Many substances possess the power of turning the plane of polarization
without the intervention of magnetism. 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. Not so with the
induced magnetic effect. Here it is manifest that the rotation would
be doubled by the act of reflection. Hence Faraday concludes that the
particles of the oil of turpentine which rotate by virtue of their
natural force, and those which rotate in virtue of the induced force,
cannot be in the same condition. The same remark applies to all bodies
which possess a natural power of rotating the plane of polarization.

And then he proceeded with exquisite skill and insight to take advantage
of this conclusion. He silvered the ends of his piece of heavy glass,
leaving, however, a narrow portion parallel to two edges diagonally
opposed to each other unsilvered. He then sent his beam through this
uncovered portion, and by suitably inclining his glass caused the beam
within it to