  Several strong resonance lines, such as H I Ly-$\alpha$, Mg II k, Ca II K, Ca
I 4227 \AA\, which are characterized by deep and broad absorption profiles in
the solar intensity spectrum, show conspicuous linear scattering polarization
signals when observed in quiet regions close to the solar limb. Such signals
show a characteristic triplet-peak structure, with a sharp peak in the line
core and extended wing lobes. The line core peak is sensitive to the presence
of magnetic fields through the Hanle effect, which however is known not to
operate in the line wings. Recent theoretical studies indicate that, contrary
to what was previously believed, the wing linear polarization signals are also
sensitive to the magnetic field through magneto-optical effects (MO). We search
for observational indications of this recently discovered physical mechanism in
the scattering polarization wings of the Ca I 4227 \AA\ line. We performed a
series of spectropolarimetric observations of this line using the Zurich
IMaging POLarimeter (ZIMPOL) camera at the Gregory-Coud\'e telescope of IRSOL
(Switzerland) and at the GREGOR telescope in Tenerife (Spain). Spatial
variations of the total linear polarization degree and of the linear
polarization angle are clearly appreciable in the wings of the observed line.
We provide a detailed discussion of our observational results, showing that the
detected variations always take place in regions where longitudinal magnetic
fields are present, thus supporting the theoretical prediction that they are
produced by MO effects.
