on their way somewhat in the manner
of sound. A sound made on one side of a metallic plate is heard on the
other side _partly_ by transmission through the plate, and _partly_ by
going around the edges, by atmospheric transmission. The new force
rays act in this manner, and Edison is said to have procured pictures
by means of the invisible agent while it was _going around the corner
_ of an opaque obstruction!

The pre-eminence of Thomas A. Edison as a scientific explorer and
inventor depends upon a quality of mind which enables him more easily
than others--more distinctly than any others--to see the touch of each
new discovery with existing conditions, and the application of it to
the problems of life. Edison catches the premonitory spark struck in
the darkness by some other master's hammer, and with that kindles a
conflagration. Though not the discoverer of the Roentgen ray, he was
able, as it would appear, to understand that discovery better even
than the discoverer. He almost immediately applied the new increment
of knowledge more successfully, we think, than any contemporary
scientist. His experimentation led him directly to the discovery of
the important fact that no photographic apparatus of any kind is
needed to enable an observer to use the X-rays in the delineation or
inspection of objects through opaque substances. He said within
himself: "Why not pass the X-rays through the object to be inspected
and then convert them into visibility, as if by fluorescence."

This scientific question Edison almost immediately solved.
Fluorescence is a property which some transparent bodies have of
producing, either on their surface or within their substance, light
different in color from that of its origin. This happens, for example,
when _green_ crystals of fluor spar afford _blue_ reflections of
light. Glass may be rendered fluorescent, as is seen in the Geisler
and Crookes tubes. Edison conceived the project of using this
phenomenon to get back the invisible rays into visibility.

The substance which he employed was the tungstate of calcium. Taking
crystals of this chemical compound, he spread the same over a cloth or
paper screen, and used that screen to catch and convert the invisible
images carried against it by the X-rays. To his surprise, his
experiment was completely successful. All that is needed in this case
is the cathode light, the object to be examined (as for instance the
hand), and the screen treated with t