  We show that for a force mediated by a vector particle coupled to a conserved
U(1) charge, the apparent range and strength can depend on the size and density
of the source, and the proximity to other sources. This "chameleon" effect is
due to screening from a light charged scalar. Such screening can weaken
astrophysical constraints on new gauge bosons. As an example we consider the
constraints on chameleonic gauged B-L. We show that although Casimir
measurements greatly constrain any B-L force much stronger than gravity with
range longer than 0.1 microns, there remains an experimental window for a long
range chameleonic B-L force. Such a force could be much stronger than gravity,
and long or infinite range in vacuum, but have an effective range near the
surface of the earth which is less than a micron.
