-waves, the
waves that affect the eye, are not the only waves carried by the ether.
Waves too short to affect the eye can affect the photographic plate, and
we can discover in this way the existence of waves only half the length
of the deep-violet waves. Still shorter waves can be discovered, until
we come to those excessively minute rays, the X-rays.

Below the Limits of Visibility

But we can extend our investigations in the other direction; we find
that the ether carries many waves longer than light-waves. Special
photographic emulsions can reveal the existence of waves five times
longer than violet-light waves. Extending below the limits of visibility
are waves we detect as heat-waves. Radiant heat, like the heat from a
fire, is also a form of wave-motion in the ether, but the waves our
senses recognise as heat are longer than light-waves. There are longer
waves still, but our senses do not recognise them. But we can detect
them by our instruments. These are the waves used in wireless
telegraphy, and their length may be, in some cases, measured in miles.
These waves are the so-called electro-magnetic waves. Light, radiant
heat, and electro-magnetic waves are all of the same nature; they differ
only as regards their wave-lengths.

LIGHT--VISIBLE AND INVISIBLE

If Light, then, consists of waves transmitted through the ether, what
gives rise to the waves? Whatever sets up such wonderfully rapid series
of waves must be something with an enormous vibration. We come back to
the electron: all atoms of matter, as we have seen, are made up of
electrons revolving in a regular orbit round a nucleus. These electrons
may be affected by out-side influences, they may be agitated and their
speed or vibration increased.

Electrons and Light

The particles even of a piece of cold iron are in a state of vibration.
No nerves of ours are able to feel and register the waves they emit, but
your cold poker is really radiating, or sending out a series of
wave-movements, on every side. After what we saw about the nature of
matter, this will surprise none. Put your poker in the fire for a time.
The particles of the glowing coal, which are violently agitated,
communicate some of their energy to the particles of iron in the poker.
They move to and fro more rapidly, and the waves which they create are
now able to affect your nerves and cause a sensation of heat. Put the
poker again in the fire, until its temperature rises to 500 deg