  Planetary nebulae are now well established as probes of galaxy dynamics and
as standard candles in distance determinations. Motivated by the need to
improve the efficiency of planetary nebulae searches and the speed with which
their radial velocities are determined, a dedicated instrument - the Planetary
Nebulae Spectrograph or PN.S - has been designed and commissioned at the 4.2m
William Herschel Telescope. The high optical efficiency of the spectrograph
results in the detection of typically ~ 150 PN in galaxies at the distance of
the Virgo cluster in one night of observations. In the same observation the
radial velocities are obtained with an accuracy of ~ 20 km/s
