e the ribs, the cartilages of the ear, and a lighter region in
the centre of the body, which marks the location of the heart.

Like most experimenters, Professor Wright has taken numerous shadow
pictures of the human hand, showing the bones within, and he has made
a great number of experiments in photographing various metals and
different varieties of quartz and glass, with a view to studying
characteristic differences in the shadows produced. A photograph
of the latter sort is reproduced on page 401. Aluminium shows a
remarkable degree of transparency to the Roentgen rays; so much so that
Professor Wright was able to photograph a medal of this metal, showing
in the same picture the designs and lettering on both sides of the
medal, presented simultaneously in superimposed images. The denser
metals, however, give in the main black shadows, which offer little
opportunity of distinguishing between them.

As to the nature of the Roentgen rays, Professor Wright is inclined
to regard them as a mode of motion through the ether, in longitudinal
stresses; and he thinks that, while they are in many ways similar to
the rays discovered by Lenard a year or so ago, they still present
important characteristics of their own. It may be, he thinks, that the
Roentgen rays are the ordinary cathode rays produced in a Crookes tube,
filtered, if one may so express it, of the metallic particles carried
in their electrical stream from the metal terminal, on passing through
the glass. It is well known that the metal terminals of a Crookes tube
are steadily worn away while the current is passing; so much so that
sometimes portions of the interior of the tube become coated with a
metallic deposit almost mirror-like.

As to the future, Professor Wright feels convinced that important
results will be achieved in surgery and medicine by the use of these
new rays, while in physical science they point to an entirely new
field of investigation. The most necessary thing now is to find some
means of producing streams of Roentgen rays of greater volume and
intensity, so as to make possible greater penetration and distinctness
in the images. Thus far only small Crookes tubes have been used, and
much is to be expected when larger ones become available; but there
is great difficulty in the manufacture of them. It might be possible,
Professor Wright thinks, to get good results by using, instead of the
Crookes tube, a large sphere of aluminium, which is more t