wood omitted. "The rays," he continued, "passed
through all the metals tested, with a facility varying, roughly
speaking, with the density of the metal. These phenomena I have
discussed carefully in my report to the Wuerzburg society, and you will
find all the technical results therein stated." He showed a photograph
of a small sheet of zinc. This was composed of smaller plates
soldered laterally with solders of different metallic proportions. The
differing lines of shadow, caused by the difference in the solders,
were visible evidence that a new means of detecting flaws and chemical
variations in metals had been found. A photograph of a compass showed
the needle and dial taken through the closed brass cover. The markings
of the dial were in red metallic paint, and thus interfered with the
rays, and were reproduced. "Since the rays had this great penetrative
power, it seemed natural that they should penetrate flesh, and so it
proved in photographing the hand, as I showed you."

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 Roentgen 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 metal