t we
now know toward these generalizations is in some measure due, and the
epoch of complete development may hardly be seen by those now alive.
It is proverbially rash to attempt prediction, but it seems to me that
it may well take a period of fifty years for these great strides to be
fully accomplished. If it does, and if progress goes on at anything
like its present rate, the aspect of physical science bequeathed to
the latter half of the twentieth century will indeed excite
admiration, and when the populace are sufficiently educated to
appreciate it, will form a worthy theme for poetry, for oratorios, and
for great works of art.

[Footnote 2: Though, indeed, a century hence it may be premature
to offer an opinion on such a point.]

To attempt to give any idea of the drift of progress in all the
directions which I have hastily mentioned, to attempt to explain the
beginnings of the theories of elasticity and of matter, would take too
long, and might only result in confusion. I will limit myself chiefly
to giving some notion of what we have gained in knowledge concerning
electricity, ether, and light. Even that is far too much. I find I
must confine myself principally to light, and only treat of the others
as incidental to that.

For now well nigh a century we have had a wave theory of light; and a
wave theory of light is quite certainly true. It is directly
demonstrable that light consists of waves of some kind or other, and
that these waves travel at a certain well-known velocity, seven times
the circumference of the earth per second, taking eight minutes on the
journey from the sun to the earth. This propagation in time of an
undulatory disturbance necessarily involves a medium. If waves setting
out from the sun exist in space eight minutes before striking our
eyes, there must necessarily be in space some medium in which they
exist and which conveys them. Waves we cannot have unless they be
waves in something.

No ordinary medium is competent to transmit waves at anything like the
speed of light; hence the luminiferous medium must be a special kind
of substance, and it is called the ether. The _luminiferous_ ether it
used to be called, because the conveyance of light was all it was then
known to be capable of; but now that it is known to do a variety of
other things also, the qualifying adjective may be dropped.

Wave motion in ether, light certainly is; but what does one mean by
the term wave? The pop