isposed of by the impossibility of seeing through a crooked pipe,
though we can hear through it; or that we cannot look round a corner,
though we can listen round one.

The present century finally established it through the discovery of
interference, the destruction of the emission theory being inevitable
when it was shown that light, interfering under certain circumstances
with light, may produce darkness, as sound added to sound may produce
silence--results arising from the action of undulating motion. The
difficulties presented by polarization were not only removed, but that
class of phenomena was actually made a strong support of the theory. The
discovery that two pencils of oppositely polarized light would not
interfere, led at once to the theory of transverse vibrations. Great
mathematical ability was now required for the treatment of the subject,
and the special consideration of many optical problems from this new
point of view, as, for example, determining the result of transverse
vibrations coming into a medium of different density in different
directions. As the theory of universal gravitation had formerly done, so
now the undulatory theory began to display its power as a physical
truth, enabling geometers to foresee results, and to precede the
experimenter in conclusions. Among earlier results of the kind was the
prediction that both the rays in the biaxial crystal topaz are
extraordinary, and that circular polarization may be produced by
reflexion in a rhomb of glass. The phenomena of depolarization offered
no special difficulty; and many new facts, as those of elliptic
polarization and conical refraction, have since illustrated the power of
the theory.

[Sidenote: The ether and its movements.] Light, then, is the result of
ethereal undulations impinging on the eye. There exists throughout the
universe and among the particles of all bodies an elastic medium, ether.
By reason of the repulsion of its own parts it is uniformly diffused in
a vacuum. In the interior of refracting media it exists in a state of
less elasticity compared with its density than in vacuo. Vibrations
communicated to it in free space are propagated through such media by
the ether in their interior. The parts of shining bodies vibrate as
those of sounding ones, communicating their movement to the ether, and
giving rise to waves in it. They produce in us the sensation of light.
The slower the vibration, the longer the wave; the more freque