  Long-baseline interferometry detects fringes created by superposition of two
beams of light collected by two telescopes pointing into a common direction.
The external path difference generated by pointing away from the zenith is
commonly compensated by adding a variable optical path length (delay) through
air for one beam such that the optical path difference between the beams
remains close to zero near the detector.
  The ABCD formula assigns a (wrapped) phase to the amplitudes A to D of an
interference pattern shifted by multiples of 90 degrees in phase. We study the
interplay between a wide band pass of the optics and the dispersion of the air
in the compensating delay, which leads to small deviations between the ABCD
phase and the reduced, monochromatic group-delay representation of the wave
packets.
  In essence, this adds dispersion to the effects that have been discussed for
evacuated interferometers (telescopes in space) before [J. Opt. Soc. Am. A 22
(2005) 2774].
