A detailed discussion of the characteristics of his rays the professor
considered unprofitable and unnecessary. He believes, though, that
these mysterious radiations are not light, because their behavior is
essentially different from that of light rays, even those light rays
which are themselves invisible. The Röntgen rays cannot be reflected
by reflecting surfaces, concentrated by lenses, or refracted or
diffracted. They produce photographic action on a sensitive film, but
their action is weak as yet, and herein lies the first important field
of their development. The professor's exposures were comparatively
long--an average of fifteen minutes in easily penetrable media,
and half an hour or more in photographing the bones of the hand.
Concerning vacuum tubes, he said that he preferred the Hittorf,
because it had the most perfect vacuum, the highest degree of air
exhaustion being the consummation most desirable. In answer to a
question, "What of the future?" he said:

"I am not a prophet, and I am opposed to prophesying. I am pursuing
my investigations, and as fast as my results are verified I shall make
them public."

"Do you think the rays can be so modified as to photograph the organs
of the human body?"

In answer he took up the photograph of the box of weights. "Here are
already modifications," he said, indicating the various degrees of
shadow produced by the aluminium, platinum, and brass weights, the
brass hinges, and even the metallic stamped lettering on the cover of
the box, which was faintly perceptible.