entirely under the spell of his doctrine, mostly preferred to
take; or by assuming that the Newtonian action at a distance is
only _apparently_ immediate action at a distance, but in truth is
conveyed by a medium permeating space, whether by movements or by
elastic deformation of this medium. Thus the endeavour toward a
unified view of the nature of forces leads to the hypothesis of an
ether. This hypothesis, to be sure, did not at first bring with it
any advance in the theory of gravitation or in physics generally,
so that it became customary to treat Newton's law of force as an
axiom not further reducible. But the ether hypothesis was bound
always to play some part in physical science, even if at first only
a latent part.

When in the first half of the nineteenth century the far-reaching
similarity was revealed which subsists between the properties of
light and those of elastic waves in ponderable bodies, the ether
hypothesis found fresh support. It appeared beyond question that
light must be interpreted as a vibratory process in an elastic, inert
medium filling up universal space. It also seemed to be a necessary
consequence of the fact that light is capable of polarisation that
this medium, the ether, must be of the nature of a solid body,
because transverse waves are not possible in a fluid, but only in
a solid. Thus the physicists were bound to arrive at the theory
of the "quasi-rigid" luminiferous ether, the parts of which can
carry out no movements relatively to one another except the small
movements of deformation which correspond to light-waves.

This theory--also called the theory of the stationary luminiferous
ether--moreover found a strong support in an experiment which is
also of fundamental importance in the special theory of relativity,