  The equations of pre-metric electromagnetism are summarized and then
formulated as an exterior differential system on the total space of the bundle
of 2-forms over the spacetime manifold. The Harrison-Estabrook method of
computing the symmetries of the system is then applied, with the result that of
the four possible formal algebras of infinitesimal symmetries, the most
physically compelling one is the Lie algebra of infinitesimal projective
transformations of real four-dimensional projective space.
