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The excerpt represents the core issue or deciding factor on which you must meditate, and is drawn from Faraday as a Discoverer by John Tyndall: 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
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
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