Fresnel perceived that this experiment absolutely compels us to reject
the hypothesis of longitudinal vibrations acting along the line of
propagation in the direction of the rays. To explain it, it must of
necessity be admitted, on the contrary, that the vibrations are
transverse and perpendicular to the ray. Verdet could say, in all
truth, "It is not possible to deny the transverse direction of
luminous vibrations, without at the same time denying that light
consists of an undulatory movement."
Such vibrations do not and cannot exist in any medium resembling a
fluid. The characteristic of a fluid is that its different parts can
displace themselves with regard to one another without any reaction
appearing so long as a variation of volume is not produced. There
certainly may exist, as we have seen, certain traces of rigidity in a
liquid, but we cannot conceive such a thing in a body infinitely more
subtle than rarefied gas. Among material bodies, a solid alone really
possesses the rigidity sufficient for the production within it of
transverse vibrations and for their maintenance during their
propagation.
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