  The nulling interferometers proposed to study planets around other stars are
generally well suited for studying small-scale structures surrounding other
bright pointlike objects such as the nuclei of active galaxies. Conventional
interferometric techniques will produce useful maps of optical/IR line and
continuum emission within active galaxies on scales of 10 milliarcseconds
(mas), but similar studies of broad-line regions will require baselines longer
than those currently envisaged. Nevertheless, nulling interferometers currently
under development will be able to constrain quasar velocity fields on
milliarcsecond scales, as long as they are equipped with spectrographs capable
of resolving lines several hundred km/s wide. This Letter describes how
analyses of line emission leaking through the edges of the null in such an
instrument can reveal the size, shape, and velocity field of nebular gas on the
outskirts of a quasar broad-line region. If this technique proves effective, it
could potentially be used to measure the mass function of quasar black holes
throughout the universe.
