  The "Magnificent Seven" (M7) are a group of radio-quiet Isolated Neutron
Stars (INSs) discovered in the soft X-rays through their purely thermal surface
emission. Owing to the large inferred magnetic fields ($B\approx 10^{13}$ G),
radiation from these sources is expected to be substantially polarised,
independently on the mechanism actually responsible for the thermal emission. A
large observed polarisation degree is, however, expected only if
quantum-electrodynamics (QED) polarisation effects are present in the
magnetised vacuum around the star. The detection of a strongly linearly
polarised signal would therefore provide the first observational evidence of
QED effects in the strong-field regime. While polarisation measurements in the
soft X-rays are not feasible yet, optical polarisation measurements are within
reach also for quite faint targets, like the M7 which have optical counterparts
with magnitudes $\approx 26$--$28$. Here, we report on the measurement of
optical linear polarisation for the prototype, and brightest member, of the
class, RX\, J1856.5$-$3754 ($V\sim 25.5$), the first ever for one of the M7,
obtained with the Very Large Telescope. We measured a polarisation degree
$\mathrm{P.D.} =16.43\% \pm5.26\%$ and a polarisation position angle
$\mathrm{P.A.}=145\fdg39\pm9\fdg44$, computed east of the North Celestial
Meridian. The $\mathrm{P.D.}$ that we derive is large enough to support the
presence of vacuum birefringence, as predicted by QED.
