ransparent
to the new rays than glass and possesses considerable strength. It is
a delicate question, however, whether the increased thickness of metal
necessary to resist the air pressure upon a vacuum would not offset
the advantage gained from the greater size. Moreover, it is a matter
for experiment still to determine, what kind of an electric current
would be necessary to excite such a larger tube with the best results.

Among the most important experiments in shadow photography made thus
far in America are those of Dr. William J. Morton of New York, who was
the first in this country to use the disruptive discharges of
static electricity in connection with the Roentgen discovery, and to
demonstrate that shadow pictures may be successfully taken without the
use of Crookes tubes. It was the well-known photographic properties of
ordinary lightning that made Dr. Morton suspect that cathode rays are
produced freely in the air when there is an electric discharge from
the heavens. Reasoning thus, he resolved to search for cathode rays in
the ten-inch lightning flash he was able to produce between the poles
of his immense Holtz machine, probably the largest in this country.

On January 30th he suspended a glass plate, with a circular window in
the middle, between the two poles. Cemented to this plate of glass was
one of hard rubber, about equal in size, which of course covered
the window in the glass. Back of the rubber plate was suspended a
photographic plate in the plate-holder, and outside of this, between
it and the rubber surface, were ten letters cut from thin copper. Dr.
Morton proposed to see if he could not prove the existence of cathode
rays between the poles by causing them to picture in shadow, upon the
sensitized plate, the letters thus exposed.

In order to do this it was necessary to separate the ordinary electric
sparks from the invisible cathode rays which, as Dr. Morton believed,
accompanied them. It was to accomplish this that he used the double
plates of glass and hard rubber placed, as already described, between
the two poles; for while the ordinary electric spark would not
traverse the rubber, any cathode rays that might be present would do
so with great ease, the circular window in the glass plate allowing
them passage there.

[Illustration: DR. WILLIAM J. MORTON PHOTOGRAPHING HIS OWN HAND UNDER
ROeNTGEN RAYS.

In this case the vacuum bulb is charged from Leyden jars which, in
their turn, are ex