  In October 1997 daily monitoring observations of the Crab pulsar at 327 MHz
and 610 MHz with an 85ft telescope in Green Bank, WV showed a jump in the
dispersion by 0.12 cm^{-3} pc. Pulses were seen simultaneously at both old and
new dispersions for a period of days. In the months before this event faint
ghost emission, a replica of the pulse, was detected with a nearly frequency
independent delay that quadratically diminished to zero. There was also a
curious shift in the phase, a slowdown, at all frequencies at the time of the
dispersion jump. I attribute most of these phenomena to the perturbing optics
of a plasma prism that is located in the filamentary interface between the
synchrotron nebula and the supernova ejecta and which crosses the line of sight
over a period of months. The required density, scale length and velocity are
reasonable given detailed HST and previous observations of these filaments.
