  We demonstrate the feasibility of determining magnitudes of stars on archival
photographic plates using a commercially available scanner. We describe one
photometric approach that could serve as a useful example for other studies. In
particular, we measure and calibrate stellar magnitudes from a 1903
photographic plate from the Yerkes Observatory collection, and demonstrate that
the overall precision from our methods is better than 0.10 mag. Notably, these
measurements are dominated by intrinsic plate noise, rather than noise
introduced through the scanning/digitization process. The low expense of this
approach expands the scientific potential to study variable stars in the
archives of observatory plate collections. We use the serendipitous discovery
of a candidate transient at photographic magnitude $pg$ = 16.60 in the spiral
galaxy NGC 7331 to illustrate our photometric methods. If this unknown source
is a supernova, it would represent the fourth known supernova in NGC 7331.
