erval which elapses between the arrival of the rays
and the attainment by the means of detection, such as the
phosphorescence of the glass or the deflection of the electrometer, of
sufficient intensity to affect the senses.

[Illustration: FIG. 25.]

_Transmission of Cathode Rays through Solids--Lenard Rays._--It was for
a long time believed that all solids were absolutely opaque to these
rays, as Crookes and Goldstein had proved that very thin glass, and even
a film of collodion, cast intensely black shadows. Hertz (_Wied. Ann._
45, p. 28), however, showed that behind a piece of gold-leaf or
aluminium foil an appreciable amount of phosphorescence occurred on the
glass, and that the phosphorescence moved when a magnet was brought
near. A most important advance was next made by Lenard (_Wied. Ann._ 51,
p. 225), who got the cathode rays to pass from the inside of a discharge
tube to the air outside. For this purpose he used a tube like that shown
in fig. 25. The cathode K is an aluminium disc 1.2 cm. in diameter
fastened to a stiff wire, which is surrounded by a glass tube. The anode
A is a brass strip partly surrounding the cathode. The end of the tube
in front of the cathode is closed by a strong metal cap, fastened in
with marine glue, in the middle of which a hole 1.7 mm. in diameter is
bored, and covered with a piece of very thin aluminium foil about .0026
mm. in thickness. The aluminium window is in metallic contact with the
cap, and this and the anode are connected with the earth. The tube is
then exhausted until the cathode rays strike against the window. Diffuse
light spreads from the window into the air outside the tube, and can be
traced in a dark room for a distance of several centimetres. From the
window, too, proceed rays which, like the cathode rays, can produce
phosphorescence, for certain bodies phosphoresce when placed in the
neighbourhood of the window. This effect is conveniently observed by the
platino-cryanide screens used to detect Rontgen radiation. The
properties of the rays outside the tube resemble in all respects those
of cathode rays; they are deflected by a magnet and by an electric
field, they ionize the gas through which they pass and make it a
conductor of electricity, and they affect a photographic plate and
change the colour of the haloid salts of the alkali metals. As, however,
it is convenient to distinguish between cathode rays outside and inside
the tube, we shall call the for