XXI
331
PASSAGE OF CATHODE RAYS THROUGH METALS
millimetres in diameter; let this aperture be closed by a piece
of aluminium leaf. If we now place a suitable glass plate
close behind the aperture we get, as might be expected, a
distinct and bright phosphorescent image of the aperture upon
the glass; but if we remove the glass plate even one or two
millimetres, the image becomes perceptibly larger and suffers
a corresponding loss of brilliancy, its edge at the same time
becoming indistinct. When the glass plate is moved back
several millimetres the image of the aperture becomes very
indistinct, large and faint; and when the plate is shifted still
further away, the tube behind the diaphragm appears quite
dark. That this is simply due to the feebleness of the cathode
rays which have been diffused from the small aperture can be
shown by introducing into the diaphragm several such
apertures closed by aluminium leaf. For this purpose the
diaphragm is best made of wire gauze hammered flat; upon
this is stretched a piece of aluminium leaf. Behind such a
diaphragm the whole of the discharge tube becomes filled
with a uniform, moderately bright light. The phosphorescence
is sufficiently strong to allow of our obtaining separate beams
by means of further diaphragms: with these we can convince
ourselves that even after passing through metallic leaf the
cathode rays retain their properties of rectilinear propagation,
of being deflected by a magnet, etc.
There must be some connection between the phenomenon
of the diffusion of cathode rays on passing through thin layers
of bright metal and another phenomenon, namely, that
when cathode rays impinge upon such a surface the portion
reflected back is diffused, as E. Goldstein has shown.¹
1 See Wiedemann's Annalen, 15, p. 246, 1882.