330
XXI
PASSAGE OF CATHODE RAYS THROUGH METALS
through openings in the metallic leaf which lies close against
it. Again, if we place two metallic leaves one on top of the
other, the number of coincident holes must become vanishingly
small. But the cathode rays are able to make glass luminesce
brightly under a double layer of metallic leaf; even under a
three or fourfold layer of gold or aluminium leaf we can per-
ceive the phosphorescence of the glass and the shadows of
objects in front of the leaf. I have been rather surprised by
the extent to which the rays are weakened by passing through
a double layer; it is much larger than one would expect from
the slight weakening produced by a single layer. I think the
following sufficiently explains this phenomenon. The metallic
layer has a reflecting surface by which the phosphorescent
light is reflected. This reflecting surface prevents the light
from radiating towards the cathode, but it doubles the intensity
of the light in the direction away from the cathode.
If we
assume that the metallic layer allows only of the cathode
rays to pass, it will not reduce the luminescence to but only
to of its previous value: whereas the second layer will reduce
it to, and further layers will soon cause the phosphor-
escence to vanish. If this conception is correct, metallic
layers capable of transmitting more than half of the cathode
rays should not weaken the luminescence at all: behind such
metallic layers the glass ought actually to phosphoresce more
strongly than in parts where it is not covered. I think I
have been able to verify this expectation in the case of layers
of silver chemically precipitated and of suitable thickness: but
the observation is not quite trustworthy, because in the un-
covered parts one cannot avoid seeing through the phosphor-
escing glass the greyish-blue luminescence of the gas, and it is
not easy to separate with any certainty the brightness of this
from that of the green phosphorescence light.
Lastly, if the cathode rays went right through the holes
in the metal they would afterwards continue their rectilinear
path. But this is just what they do not do; by their passage
through the metal they become diffused, just as light does by
passing through a turbid medium such as milk glass. Let
part of a cylindrical discharge tube be shut off, say at a
distance of 20 cm. from the cathode, by a metal plate extend-
ing right across it but containing a circular aperture a few