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 behaviour 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 aluminum, platinum, and brass weights, the brass
hinges, and even the metall