  We describe a new, deep photographic survey of the Virgo Cluster which uses
multiple exposures on Tech Pan film with the United Kingdom Schmidt Telescope
to probe the dwarf population to fainter surface brightness limits than
previous surveys. We have identified galaxies having sizes (>= 3 arcsec scale
length) and surface brightnesses (<= 24.5 R mag arcsec^-2) characteristic of
those expected for dwarf spheroidal galaxies in the cluster. The survey is
providing substantial samples of extremely low luminosity galaxies outside the
environment of the Local Group and nearby groups for the first time. An initial
study of two small areas has found dwarf spheroidal candidates in large numbers
(500 deg^-2) which indicate a steep, continuously rising luminosity function at
low luminosities.
