  We realized a laboratory experiment to study the scattering polarization of
the Na I D-doublet at 589.0 and 589.6 nm in the presence of a magnetic field.
This work was stimulated by solar observations of that doublet, which have
proven particularly challenging to explain through available models of
polarized line formation, even to the point of casting doubts on our very
understanding of the underlying physics. The purpose of the experiment was to
test a quantum theory for the polarized scattering of spectrally flat incident
radiation, on which much of the current magnetic diagnostics of stellar
atmospheres is based. The experiment has confirmed the predictions of that
theory, and its adequacy for the modeling of scattering polarization under
flat-spectrum illumination.
