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.
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