  HST observations of some QSOs show a strong, abrupt increase in polarization
at rest wavelength about 750 A, suggestive of a connection with the Lyman edge
of hydrogen. Blaes and Agol (1996) have proposed an explanation in terms of
stellar atmosphere effects in an accretion disk around a supermassive black
hole. We have computed the polarized spectrum of a such a disk, including the
effects of the relativistic transfer function. Relativistic effects add an
additional blueshift of the polarization rise sufficient that the model cannot
explain the observations.
  A good fit results if the emitted radiation is assumed to have a sharp
increase in polarized flux at the Lyman edge in the rest frame of the orbiting
gas. Relativistic effects then cause the observed polarization to rise sharply
at a wavelength substantially less than 912 A. The blueshift depends on the
angular momentum of the black hole and the inclination of the disk. A good fit
to PG 1630+377 results from a simple model with a dimensionless angular
momentum a = 0.5 and an observer viewing angle cos theta = 0.1. An intermediate
value of a might result from coallescing black holes, successive accretion
events, or electromagnetic extraction of angular momentum from the hole.
