324
XX
LIGHT AND ELECTRICITY
in air, the velocity of electric waves is enormously great, so
that we
can make a direct comparison between the two.
Now the velocity of electric waves in wires has long since
been directly measured. This was an easier problem to solve,
because such waves can be followed for several kilometres.
Thus we obtain another measurement, purely experimental, of
our velocity, and if the result is only an approximate one it at
any rate does not contradict the first.
All these experiments in themselves are very simple, but
they lead to conclusions of the highest importance. They are
fatal to any and every theory which assumes that electric
force acts across space independently of time. They mark a
brilliant victory for Maxwell's theory. No longer does this
connect together natural phenomena far removed from each
other. Even those who used to feel that this conception as
to the nature of light had but a faint air of probability now
find a difficulty in resisting it. In this sense we have reached
our goal. But at this point we may perhaps be able to do
without the theory altogether. The scene of our experiments
was laid at the summit of the pass which, according to the
theory, connects the domain of optics with that of electricity.
It was natural to go a few steps further, and to attempt the
descent into the known region of optics. There may be some
advantage in putting theory aside. There are many lovers of
science who are curious as to the nature of light and are
interested in simple experiments, but to whom Maxwell's
theory is nevertheless a seven-sealed book. The economy of
science, too, requires of us that we should avoid roundabout
ways when a straight path is possible. If with the aid of our
electric waves we can directly exhibit the phenomena of light,
we shall need no theory as interpreter; the experiments them-
selves will clearly demonstrate the relationship between the
two things. As a matter of fact such experiments can be
performed. We set up the conductor in which the oscillations
are excited in the focal line of a very large concave mirror.
The waves are thus kept together and proceed from the mirror
as a powerful parallel beam. We cannot indeed see this beam
directly, or feel it; its effects are manifested in exciting sparks
in the conductors upon which it impinges. It only becomes
visible to our eyes when they are armed with our resonators.