e of the papers the additional
weight is sufficient to depress one of the pans as shown in the second
photograph. The spectroscope will detect less than one-millionth of the
matter contained in the word pencilled above.]

[Illustration: _Reproduced by permission of X-Rays Ltd._

THIS X-RAY PHOTOGRAPH IS THAT OF A HAND OF A SOLDIER WOUNDED IN THE
GREAT WAR

Note the pieces of shrapnel which are revealed.]

[Illustration: _Photo: National Physical Laboratory._

AN X-RAY PHOTOGRAPH OF A GOLF BALL, REVEALING AN IMPERFECT CORE]

[Illustration: _Reproduced by permission of X-Rays Ltd._

A WONDERFUL X-RAY PHOTOGRAPH

Note the fine details revealed, down to the metal tags of the bootlace
and the nails in the heel of the boot.]

Sec. 4

The Discovery of X-rays

So the story went on from year to year. We shall see in a moment to what
it led. Meanwhile the next great step was when, in 1895, Roentgen
discovered the X-rays, which are now known to everybody. He was
following up the work of Lenard, and he one day covered a "Crookes tube"
with some black stuff. To his astonishment a prepared chemical screen
which was near the tube began to glow. _The rays had gone through the
black stuff; and on further experiment he found that they would go
through stone, living flesh, and all sorts of "opaque" substances._ In a
short time the world was astonished to learn that we could photograph
the skeleton in a living man's body, locate a penny in the interior of a
child that had swallowed one, or take an impression of a coin through a
slab of stone.

And what are these X-rays? They are not a form of matter; they are not
material particles. X-rays were found to be a new variety of _light_
with a remarkable power of penetration. We have seen what the
spectroscope reveals about the varying nature of light wave-lengths.
Light-waves are set up by vibrations in ether,[2] and, as we shall see,
these ether disturbances are all of the same kind; they only differ as
regards wave-lengths. The X-rays which Roentgen discovered, then, are
light, but a variety of light previously unknown to us; they are ether
waves of very short length. X-rays have proved of great value in many
directions, as all the world knows, but that we need not discuss at this
point. Let us see what followed Roentgen's discovery.

[2] We refer throughout to the "ether" because, although modern
theories dispense largely with this conception, the theories of
p