  Radiation pressure measurements on mirrors submerged in dielectric liquids
have consistently shown an effective Minkowski momentum for the photons within
the liquid. Using an exact theoretical calculation based on Maxwell's equations
and the Lorentz law of force, we demonstrate that this result is a consequence
of the fact that conventional mirrors impart, upon reflection, a 180 degree
phase-shift to the incident beam of light. If the mirror is designed to impart
a different phase, then the effective momentum will turn out to be anywhere
between the two extremes of the Minkowski and Abraham momenta. Since all values
in the range between these two extremes are equally likely to be found in
experiments, we argue that the photon momentum inside a dielectric host has the
arithmetic mean value of the Abraham and Minkowski momenta.
