iments are considerably larger than any
I have seen elsewhere, being pear-shaped, about eight inches long, and
four inches wide at the widest part. It is, perhaps, to the excellence
of this tube that Dr. Robb owes part of his success. At any rate, in
the foot picture the bones are outlined through shoe and stocking,
while every nail in the sole of the shoe shows plainly, although the
rays came from above, striking the top of the foot first, the sole
resting upon the plate-holder. In other of Dr. Robb's pictures equally
fine results were obtained; notably in one of a fish, reproduced
herewith, and showing the bony structure of the body; one of a razor,
where the lighter shadow proves that the hollow ground portion is
almost as thin as the edge; and one of a man's hand, taken for use
in a lawsuit, to prove that the bones of the thumb, which had been
crushed and broken in an accident, had been improperly set by the
attending physician.

[Illustration: THOMAS A. EDISON EXPERIMENTING WITH THE ROeNTGEN RAYS.]

Dr. Robb has made a series of novel and important experiments with
tubes from which the air has been exhausted in varying degrees, and
has concluded from these that it is impossible to produce the Roentgen
phenomena unless there is present in the tube an almost perfect
vacuum. Through a tube half exhausted, on connecting it with an
induction coil, he obtained merely the ordinary series of sparks; in a
tube three-quarters exhausted, he obtained a reddish glow from end to
end, a torpedo-shaped stream of fire; through a tube exhausted to
a fairly high degree--what the electric companies would call "not
bad"--he obtained a beautiful steaked effect of bluish striae in
transverse layers. 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