In the appendix to Space TIme and Gravitation, V. Fock demostrated that singular
solutions to Maxwell's Equations of electrodynamics satisfied the eikonal
expression, a quadratic partial differential equation with signature {+++}.
Mappings which preserved the eikonal, taking a discontinuity in E field to a
discontinuity, were of two and only two types. A linear type which Fock proved
was the Lorentz group of transformations . This result is the foundation of
special relativity.
The other mapping was a nonlinear Mobius projective transformation. In the
linear mapping, it can be argued that the propagation speed of the singular
solutions must be a constant. (The ubiquitous c  the speed of light). For the
nonlinear mapping the propagation speed of the singularity can be anything 
including infinity. !!!
In optically active media, the propagation speed of the discontinuities is
faster or slower that the speed of light, depending on the whether or not the
helicity (circular polarization) is aligned or antialigned with the optical
axis.
