  To consider a medium carrying light and electromagnetic waves is impossible,
when this medium shall have properties according to the principle of constant
speed of light, that is, isotropy of speed of light in every system of
reference. Therefore, with principle of constant speed of light abandoned, the
so-called medium synchronization of clocks is defined, yielding isotropy solely
in the system at rest and anisotropy in all moving systems. From medium
synchronization the appropriate coordinate transformation is developed, the
so-called medium transformation, a Galilean type of transformation, thus
maintaining simultaneity between all participant systems of reference and
changing the form of Maxwells equations to the extended Maxwell equations.
Despite the fact that medium transformation violates Lorentz invariance and the
principle of relativity, it is demonstrated that the results from medium
transformation and extended Maxwell equations are fully compliant with
observable phenomena. Moreover, it is shown that the concept of a medium, a
preferred frame of reference, is compatible not only with medium
transformation, but also with Lorentz transformation and with theory of special
relativity, so it is quite possible now to consider a realistic medium.
Finally, a discrepancy is exposed within theory of special relativity, giving
rise to assume a preferred frame of reference even in special relativity.
