ght.

"Invisible light," he answered. "I do not mean the Roentgen ray, nor the
emanation from radium, both of which are invisible, but neither of
which is light, in that neither can be reflected nor refracted. Both
will penetrate many different kinds of matter, but it needs reflection
or refraction to make visible an object on which it impinges.
Understand?"

"Hardly," I answered dubiously. "What kind of visible light is there,
if not radium or the Roentgen ray? You can photograph with either, can't
you?"

"Yes, but to see what you have photographed you must develop the film.
And there is no time for that aboard a fast steamer running through the
ice and the fog. No, it is mere theory, but I have an idea that the
ultraviolet light--the actinic rays beyond the violet end of the
spectrum, you know--will penetrate fog to a great distance, and in
spite of its higher refractive power, which would distort and magnify
an object, it is better than nothing."

"But what makes you think that it will penetrate fog?" I queried. "And
if it is invisible itself, how will it illumine an object?"

"As to your first question," he answered, with a smile, "it is well
known to surgeons that ultraviolet light will penetrate the human body
to the depth of an inch, while the visible rays are reflected at the
surface. And it has been known to photographers for fifty years that
this light--easily isolated by dispersion through prisms--will act on a
sensitized plate in an utterly dark room."

"Granted," I said. "But how about the second question? How can you see
by this light?"

"There you have me," he answered. "It will need a quicker development
than any now known to photography--a traveling film, for instance, that
will show the picture of an iceberg or a ship before it is too late to
avoid it--a traveling film sensitized by a quicker acting chemical than
any now used."

"Why not puzzle it out?" I asked. "It would be a wonderful invention."

"I am too old," he answered dreamily. "My life work is about done. But
other and younger men will take it up. We have made great strides in
optics. The moving picture is a fact. Colored photographs are possible.
The ultraviolet microscope shows us objects hitherto invisible because
smaller than the wave length of visible light. We shall ultimately use
this light to see through opaque objects. We shall see colors never
imagined by the human mind, but which have existed since the beginning
of ligh