  The response of a pair of differently polarized antennas is determined by
their polarization states AND a phase between them which has a geometric part
which becomes discontinuous at singular points in the parameter space. Such
phase singularities have been observed in optical polarization experiments
[R.Bhandari, Physics Reports 281 (1997) 1]. It is pointed out that such a phase
is implicit in an equation for the response derived in the sixties [Morris et
al Ap.J. 139 (1964) 551].
  When a source emitting partially polarized radiation is observed with a
correlation interferometer consisting of two antennas with different
polarizations, curiously, fringes can be made to vanish for an appropriate
choice of the polarization states, in general non orthogonal [V Radhakrishnan,
Current Science 67 (1994) 257]. The result follows by considering the
interference as a superposition of two patterns due to the two orthogonal
polarization components of the radiation, which have a phase difference of
magnitude \pi, a geometric phase.
