  We have combined high resolution magneto-optical imaging with an ultra-fast
heating/cooling technique to measure the movement of individual vortices in a
superconducting film. The motion took place while the film was heated close to
$T_c$, where pinning and viscous forces are relatively small. Under these
conditions, vortices move due to the magnetic repulsion between them. We found
that a finite vortex mass has to be included in the analysis in order to
account for the experimental results. The extent of the motion is consistent
with a vortex mass being 3 orders of magnitude smaller than the mass of all the
electrons in the core.
