cathode rays produced a green phosphorescence, this plate is made to
glow in a peculiar manner. The rays producing this glow were not the
cathode rays, although apparently arising from them, and are what have
since been called the Roentgen rays, or X-rays.

Roentgen found that a shadow is thrown upon the screen by substances
held between it and the exhausted tube, the character of the shadow
depending upon the density of the substance. Thus metals are almost
completely opaque to the rays; such substances as bone much less so, and
ordinary flesh hardly so at all. If a coin were held in the hand that
had been interposed between the tube and the screen the picture formed
showed the coin as a black shadow; and the bones of the hand, while
casting a distinct shadow, showed distinctly lighter; while the soft
tissues produced scarcely any shadow at all. The value of such a
discovery was obvious from the first; and was still further enhanced by
the discovery made shortly that, photographic plates are affected by the
rays, thus making it possible to make permanent photographic records of
pictures through what we know as opaque substances.

What adds materially to the practical value of Roentgen's discovery is
the fact that the apparatus for producing the X-rays is now so simple
and relatively inexpensive that it is within the reach even of amateur
scientists. It consists essentially of an induction coil attached either
to cells or a street-current plug for generating the electricity, a
focus tube, and a phosphorescence screen. These focus tubes are made in
various shapes, but perhaps the most popular are in the form of a glass
globe, not unlike an ordinary small-sized water-bottle, this tube being
closed and exhausted, and having the two poles (anode and cathode)
sealed into the glass walls, but protruding at either end for attachment
to the conducting wires from the induction coil. This tube may be
mounted on a stand at a height convenient for manipulation.
The phosphorescence screen is usually a plate covered with some
platino-cyanide and mounted in the end of a box of convenient size, the
opposite end of which is so shaped that it fits the contour of the face,
shutting out the light and allowing the eyes of the observer to focalize
on the screen at the end. For making observations the operator has
simply to turn on the current of electricity and apply the screen to
his eyes, pointing it towards the glowing tube, when the sh