ed
both by Tesla and Elihu Thomson, who, separately and independently of
each other, have produced excellent forms of high-frequency
induction coils.

Induction coils have long been in use for purposes of research, and in
later years have been employed in the production both of the Roentgen
rays used in the photography of the invisible, and the electro-magnetic
waves used in wireless telegraphy.

Roentgen's discovery was published in 1895. It was rendered possible by
the prior work of Geissler and Crookes on the luminous phenomena
produced by the passage of electric discharges through high vacua in
glass tubes. Roentgen discovered that the invisible rays, or radiation,
emitted from certain parts of a high-vacuum tube, when high-tension
discharges from induction coils were passing, possessed the curious
property of traversing certain opaque substances as readily as light
does glass or water. He also discovered that these rays were capable of
exciting fluorescence in some substances,--that is, of causing them to
emit light and become luminous,--and that these rays, like the rays of
light, were capable of affecting a photographic plate. From these
properties two curious possibilities arose; namely, to see through
opaque bodies, and to photograph the invisible. Roentgen called these
rays X, or unknown rays. They are now almost invariably called by the
name of their distinguished discoverer.

Let us briefly investigate how it is possible both to see and to
photograph the invisible. Shortly after Roentgen's discovery, Edison,
with that wonderful power of finding practical applications for nearly
all discoveries, had invented the fluoroscope,--a screen covered with a
peculiar chemical substance that becomes luminous when exposed to the
Roentgen rays. Suppose, now, between the rays and such a screen be
interposed a substance opaque to ordinary light, as, for example, the
human hand. The tissues of the hand, such as the flesh and the blood,
permit the rays to readily pass through them, but the bones are opaque
to the rays, and, therefore, oppose their passage; consequently, the
screen; instead of being uniformly illumined, will show shadows of the
bones, so that, to an eye examining the screen, it will seem as though
it were looking through the flesh and blood directly at the bones. In a
similar manner, if a photographic plate be employed instead of the
screen, a distinct photographic picture will be obtained.

Both the flu