  A recent proposal to measure the proton form factor by means of muon-proton
scattering will use muons which are not ultrarelativistic (and also not
nonrelativistic). The usual equations describing the scattering cross section
use the approximation that the scattered lepton (usually an electron) is
ultrarelativistic, with v/c approximately equal to 1. Here the cross section is
calculated for all values of the energy. It agrees with the standard result in
the appropriate limit.
