ABSTRACT
One of the chief differences between light and cathode rays lies in their power to pass through solid bodies. The very substances which are most transparent to all kinds of light offer an insuperable resistance to the passage of cathode rays. Commercial aluminum leaf seems to work best. It is almost completely opaque to light but very transparent to the cathode rays; it is easily handled, and is not attacked by the cathode rays, whereas a layer of silver leaf is soon corroded by them. It might be urged, against the assumption that the cathode rays in simple experiments penetrate right through the substance of the metal, that such thin metallic layers are full of small holes, and that the cathode rays might well reach the glass through these without going through the metal itself. There must be some connection between the phenomenon of the diffusion of cathode rays on passing through thin layers of bare metal and another phenomenon.
