  We describe observations of the white-light structures in the low corona
following the X8.2 flare SOL2017-09-10, as observed in full Stokes parameters
by the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics
Observatory. These data show both bright loops and a diffuse emission region
above them. We interpret the loops as the white-light counterpart of a
classical loop-prominence system, intermediate between the hot X-ray loops and
coronal rain. The diffuse emission external to the loops is linearly polarized
and has a natural interpretation in terms of Thomson scattering from the hot
plasma seen prior to its cooling and recombination. The polarimetric data from
HMI enable us to distinguish this contribution of scattering from the HMI
pseudo-continuum measurement, and to make a direct estimation of the coronal
mass in the polarized source. For a snapshot at 16:19~UT, we estimate a mass $8
\times 10^{14}$~g. We further conclude that the volumetric filling factor of
this source is near unity.
