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 accuracy the exact proportionality of the
rotation to the distance traversed by the polarized beam. Thus in one
series of experiments where the rotation required by the direct beam
was 12degrees, that acquired by three passages through the glass was
36degrees, while that acquired by five passages was 60degrees. But even
when this method of magnifying was applied, he failed with various
solid substances to obtain any effect; and in the case of air, though he
employed to the utmost the power which these repeated reflections placed
in his hands, he failed to produce the slightest sensible rotation.

These failures of Faraday to obtain the effect with gases seem to
indicate the true seat of the phenomenon. The luminiferous ether
surrounds and is influenced by the ultimate particles of matter. The
symmetry of the one involves that of the other. Thus, if the molecules
of a crystal be perfectly symmetrical round any line through the
crystal, we may safely conclude that a ray will pass along this line
as through ordinary glass. It will not be doubly refracted. From the
symmetry of the liquid figures, known to be produced in the planes of
freezing, when radiant heat is sent through ice, we may safely infer
symmetry of aggregation, and hence conclude that the line perpendicular
to the planes of freezing is a line of no double refraction; that it is,
in fact, the optic axis of the crystal. The same remark applies to the
line joining the opposite blunt angles of a crystal of Iceland spar.
The arrangement of the molecules round this line being symmetrical,
the condition of the ether depending upon these molecules shares their
symmetry; and there is, therefore, no reason why the wavelength should
alter with the alteration of the azimuth round this line. Annealed glass
has its molecules symmetrically arranged round every line that can
be drawn through it; hence it is not doubly refractive. But let the
substance be either squeezed or strained in one direction, the molecular
symmetry, and with it the symmetry of the ether, is immediately
destroyed and the glass becomes doubly refractive. Unequal heating
produces the same effect. Thus mechanical strains reveal themselves by
optical effects; and there is little doubt that