XXI
ON THE PASSAGE OF CATHODE RAYS THROUGH
THIN METALLIC LAYERS
(Wiedemann's Annalen, 45, pp. 28-32, 1892.)
ONE of the chief differences between light and cathode rays is
in respect of their power of passing through solid bodies. The
very substances which are most transparent to all kinds of
light offer, even in the thinnest layers which can be prepared,
an insuperable resistance to the passage of cathode rays. I have
been all the more surprised to find that metals, which are opaque
to light, are slightly transparent to cathode rays. Metallic
layers of moderate thickness are of course as opaque to cathode
rays as they are to light. But if a metallic layer is so thin
as to allow a part of the incident light to pass through, it will
also allow a part of the incident cathode rays to pass through;
and the proportion transmitted appears to be larger in the
latter than in the former case. This can be demonstrated by
a very simple experiment. Take a plane glass plate capable
of phosphorescing, best a piece of uranium glass: partially
cover one side, which we shall call the front side, with
pure gold leaf, and in front of this fasten a piece of mica.
Expose this front side to cathode rays proceeding from a flat
circular aluminium cathode of 1 cm. diameter, say at a dis-
tance of 20 cm. from the cathode. So long as the ex-
haustion is but moderate the cathode rays fill the whole of
the discharge tube as a powerful cone of light, and the glass
only phosphoresces outside the patch which is covered with
gold. At this stage the phosphorescence is chiefly caused by
the light of the discharge, and only a very small part of this