. C. It
begins to glow with a dull red. Its particles are now moving very
violently, and the waves they send out are so short and rapid that they
can be picked up by the eye--we have _visible_ light. They would still
not affect a photographic plate. Heat the iron further, and the crowds
of electrons now send out waves of various lengths which blend into
white light. What is happening is the agitated electrons flying round in
their orbits at a speed of trillions of times a second. Make the iron
"blue hot," and it pours out, in addition to light, the _invisible_
waves which alter the film on the photographic plate. And beyond these
there is a long range of still shorter waves, culminating in the X-rays,
which will pass between the atoms of flesh or stone.

Nearly two hundred and fifty years ago it was proved that light
travelled at least 600,000 times faster than sound. Jupiter, as we saw,
has moons, which circle round it. They pass behind the body of the
planet, and reappear at the other side. But it was noticed that, when
Jupiter is at its greatest distance from us, the reappearance of the
moon from behind it is 16 minutes and 36 seconds later than when the
planet is nearest to us. Plainly this was because light took so long to
cover the additional distance. The distance was then imperfectly known,
and the speed of light was underrated. We now know the distance, and we
easily get the velocity of light.

No doubt it seems far more wonderful to discover this within the walls
of a laboratory, but it was done as long ago as 1850. A cogged wheel is
so mounted that a ray of light passes between two of the teeth and is
reflected back from a mirror. Now, slight as is the fraction of a second
which light takes to travel that distance, it is possible to give such
speed to the wheel that the next tooth catches the ray of light on its
return and cuts it off. The speed is increased still further until the
ray of light returns to the eye of the observer through the notch _next_
to the one by which it had passed to the mirror! The speed of the wheel
was known, and it was thus possible again to gather the velocity of
light. If the shortest waves are 1/67,000 of an inch in length, and
light travels at 186,000 miles a second, any person can work out that
about 800 trillion waves enter the eye in a second when we see "violet."

Sorting out Light-waves

The waves sent out on every side by the energetic electrons become
faintly visibl