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
in Faraday's experiment it is the magnetic strain that produces the
rotation of the plane of polarization.[2]

Footnotes to Chapter 10

[1] 'By a diamagnetic,' says Faraday, 'I mean a body through which
lines of magnetic force are passing, and which does not by their
action assume the usual magnetic state of iron or loadstone.'
Faraday subsequently used this term in a different sense from that
here given, as will immediately appear.

[2] The power of double refraction conferred on the centre of a
glass rod, when it is caused to sound the fundamental note due to
its longitudinal vibration, and the absence of the same power in the
case of vibrating air (enclosed in a glass organ-pipe), seems to be
analogous to the presence and absence of Faraday's effect in the
same two substances.

Faraday never, to my knowledge, attempted to give, even in
conversation, a picture of the molecular condition of his heavy
glass when subjected to magnetic influence. In a mathematical
investigation of the subject, published in the Proceedings of the
Royal Society for 1856, Sir William Thomson arrives at the
conclusion that the 'diamagnetic' is in a state of molecular
rotation.