  We generate tightly focused optical vector beams whose electric fields spin
around an axis transverse to the beams' propagation direction. We
experimentally investigate these fields by exploiting the directional
near-field interference of a dipole-like plasmonic field probe, placed adjacent
to a dielectric interface, which depends on the transverse electric spin
density of the excitation field. Near- to far-field conversion mediated by the
dielectric interface enables us to detect the directionality of the emitted
light in the far-field and, therefore, to measure the transverse electric spin
density with nanoscopic resolution. Finally, we determine the longitudinal
electric component of Belinfante's elusive spin momentum density, a solenoidal
field quantity often referred to as 'virtual'.
