  Following Kottler, \'E.Cartan, and van Dantzig, we formulate the Maxwell
equations in a metric independent form in terms of the field strength $F=(E,B)$
and the excitation $H=({\cal D}, {\cal H})$. We assume a linear constitutive
law between $H$ and $F$. First we split off a pseudo-scalar (axion) field from
the constitutive tensor; its remaining 20 components can be used to define a
duality operator $^#$ for 2-forms. If we enforce the constraint $^{##}=-1$,
then we can derive of that the conformally invariant part of the {\em metric}
of spacetime.
