Shadow. Electric.
A term applied to a phenomenon of high vacua. If an electric discharge
is maintained in a Crookes' tube the glass opposite the negative
electrode tends to phosphoresce. A plate of aluminum, used also as the
positive electrode, protects the glass directly behind it so as to
produce the effect of a shadow.
Synonym--Molecular Shadow.
[Transcriber's note: The effect is due to the "shadowing" of the
electrons streaming past the plate.]