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 reach his eye first direct, and
then after two, four, and six reflections. These corresponded to
the passage of the ray once, three times, five times, and seven
times through the glass. He thus established with numerical