imply
because we have no intermediate sense organ to detect rates of
vibration between 40,000 and 4,000,000,000,000,000 per second. It was,
therefore, an unexplored territory. Waves have been there all the time
in any quantity, but we have not thought about them nor attended to
them.

It happens that I have myself succeeded in getting electric
oscillations so slow as to be audible. The lowest I have got at
present are 125 per second, and for some way above this the sparks
emit a musical note; but no one has yet succeeded in directly making
electric oscillations which are visible, though indirectly every one
does it when they light a candle.

Here, however, is an electric oscillator, which vibrates 300 million
times a second, and emits ethereal waves a yard long. The whole range
of vibrations between musical tones and some thousand million per
second is now filled up.

These electro-magnetic waves have long been known on the side of
theory, but interest in them has been immensely quickened by the
discovery of a receiver or detector for them. The great though simple
discovery by Hertz of an "electric eye," as Sir W. Thomson calls it,
makes experiments on these waves for the first time easy or even
possible. We have now a sort of artificial sense organ for their
appreciation--an electric arrangement which can virtually "see" these
intermediate rates of vibration.

The Hertz receiver is the simplest thing in the world--nothing but a
bit of wire or a pair of bits of wire adjusted so that when immersed
in strong electric radiation they give minute sparks across a
microscopic air gap.

The receiver I have here is adapted for the yard-long waves emitted
from this small oscillator; but for the far longer waves emitted by a
discharging Leyden jar an excellent receiver is a gilt wall paper or
other interrupted metallic surface. The waves falling upon the
metallic surface are reflected, and in the act of reflection excite
electric currents, which cause sparks. Similarly, gigantic solar waves
may produce aurorae; and minute waves from a candle do electrically
disturb the retina.

The smaller waves are, however, far the most interesting and the most
tractable to ordinary optical experiments. From a small oscillator,
which may be a couple of small cylinders kept sparking into each other
end to end by an induction coil, waves are emitted on which all manner
of optical experiments can be performed.

They can be reflected by p