cited by an induction coil.]

The current being turned on, it was found that the powerful electric
sparks visible to the eye, unable to follow a straight course on
account of the intervening rubber plate, jumped around the two plates
in jagged, lightning-like lines, and thus reached the other pole of
the machine. But it was noticed that at the same time a faint spray of
purplish light was streaming straight through the rubber between the
two holes, as if its passage was not interfered with by the rubber
plate. It was in company with this stream of violet rays, known as the
brush discharge, that the doctor conceived the invisible Roentgen
rays to be projected at each spark discharge around the plate; and
presently, when the photographic plate was developed, it was found
that his conception was based on fact. For there, dim in outline, but
unmistakable, were shadow pictures of the ten letters which stand as
historic, since they were probably the first shadow pictures in the
world taken without any bulb or vacuum tube whatever. These shadow
pictures Dr. Morton carefully distinguished from the ordinary
blackening effects on the film produced by electrified objects.

Pursuing his experiments with static electricity, Dr. Morton soon
found that better results could be obtained by the use of Leyden jars
influenced by the Holtz machine, and discharging into a vacuum bulb,
as shown in the illustration on this page. This arrangement of the
apparatus has the advantage of making it much easier to regulate the
electric supply and to modify its intensity, and Dr. Morton finds that
in this way large vacuum tubes, perhaps twenty inches in diameter,
may be excited to the point of doing practical work without danger of
breaking the glass walls. But certain precautions are necessary. When
he uses tin-foil electrodes on the outside of the bulb, he protects
the tin-foil edges, and, what is more essential, uses extremely small
Leyden jars and a short spark gap between the poles of the discharging
rods. The philosophy of this is, that the smaller the jars, the
greater their number of oscillations per second (easily fifteen
million, according to Dr. Lodge's computations), the shorter the wave
length, and, therefore, the greater the intensity of effects.

[Illustration: A GROUP OF FAMILIAR ARTICLES UNDER THE ROeNTGEN RAYS.

From a photograph by Professor Arthur W. Wright of Yale College, taken
through an ebonite plate-holder with fifty-five m