le something Young rechristened the Luminiferous Ether.

In the early days of his discovery Young thought of the undulations
which produce light and radiant heat as being longitudinal--a forward
and backward pulsation, corresponding to the pulsations of sound--and as
such pulsations can be transmitted by a fluid medium with the properties
of ordinary fluids, he was justified in thinking of the ether as being
like a fluid in its properties, except for its extreme intangibility.
But about 1818 the experiments of Fresnel and Arago with polarization
of light made it seem very doubtful whether the theory of longitudinal
vibrations is sufficient, and it was suggested by Young, and
independently conceived and demonstrated by Fresnel, that the
luminiferous undulations are not longitudinal, but transverse; and all
the more recent experiments have tended to confirm this view. But it
happens that ordinary fluids--gases and liquids--cannot transmit lateral
vibrations; only rigid bodies are capable of such a vibration. So
it became necessary to assume that the luminiferous ether is a body
possessing elastic rigidity--a familiar property of tangible solids, but
one quite unknown among fluids.

The idea of transverse vibrations carried with it another puzzle. Why
does not the ether, when set aquiver with the vibration which gives us
the sensation we call light, have produced in its substance subordinate
quivers, setting out at right angles from the path of the original
quiver? Such perpendicular vibrations seem not to exist, else we might
see around a corner; how explain their absence? The physicist could
think of but one way: they must assume that the ether is incompressible.
It must fill all space--at any rate, all space with which human
knowledge deals--perfectly full.

These properties of the ether, incompressibility and elastic rigidity,
are quite conceivable by themselves; but difficulties of thought appear
when we reflect upon another quality which the ether clearly
must possess--namely, frictionlessness. By hypothesis this rigid,
incompressible body pervades all space, imbedding every particle of
tangible matter; yet it seems not to retard the movements of this matter
in the slightest degree. This is undoubtedly the most difficult to
comprehend of the alleged properties of the ether. The physicist
explains it as due to the perfect elasticity of the ether, in virtue
of which it closes in behind a moving particle with a push e