Finally, in a tube exhausted as highly as possible,
he obtained a faint fluorescent glow, like that produced in a
Crookes tube. This fluorescence of the glass, according to Dr. Robb,
invariably accompanies the discharge of Roentgen rays, and it is likely
that these rays are produced more abundantly as the fluorescence
increases. Just how perfect a vacuum is needed to give the best
results remains a matter of conjecture. It is possible, of course,
as Tesla believes, that with an absolutely perfect vacuum no results
whatever would be obtained.
Dr. Robb has discovered that in order to get the best results with
shadow pictures it is necessary to use special developers for the
plates, and a different process in the dark-room from the one known
to ordinary photographers. In a general way, it is necessary to
use solutions designed to affect the ultra-violet rays, and not the
visible rays of the spectrum. Having succeeded, after much experiment,
in thus modifying his developing process to meet the needs of the
case, Dr. Robb finds that he makes a great gain in time of exposure,
fifteen minutes being sufficient for the average shadow picture taken
through a layer of wood or leather, and half an hour representing an
extreme case. In some shadow pictures, as, for instance, in taking a
lead-pencil, it is a great mistake to give an exposure exceeding two
or three minutes; for the wood is so transparent that with a long
exposure it does not show at all, and the effect of the picture is
spoiled.
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