http://www.fourmilab.ch/etexts/einstein/specrel/www/ :
ON THE ELECTRODYNAMICS OF MOVING BODIES
By A. Einstein June 30, 1905
"It is known that Maxwell's electrodynamics--as usually understood at
the present time--when applied to moving bodies, leads to asymmetries
which do not appear to be inherent in the phenomena. Take, for example,
the reciprocal electrodynamic action of a magnet and a conductor. The
observable phenomenon here depends only on the relative motion of the
conductor and the magnet, whereas the customary view draws a sharp
distinction between the two cases in which either the one or the other
of these bodies is in motion. For if the magnet is in motion and the
conductor at rest, there arises in the neighbourhood of the magnet an
electric field with a certain definite energy, producing a current at
the places where parts of the conductor are situated. But if the magnet
is stationary and the conductor in motion, no electric field arises in
the neighbourhood of the magnet. In the conductor, however, we find an
electromotive force, to which in itself there is no corresponding
energy, but which gives rise--assuming equality of relative motion in
the two cases discussed--to electric currents of the same path and
intensity as those produced by the electric forces in the former case.
Examples of this sort, together with the unsuccessful attempts to
discover any motion of the earth relatively to the ``light medium,''
suggest that the phenomena of electrodynamics as well as of mechanics
possess no properties corresponding to the idea of absolute rest. They
suggest rather that, as has already been shown to the first order of
small quantities, the same laws of electrodynamics and optics will be
valid for all frames of reference for which the equations of mechanics
hold good."

Examples of this sort also suggest that the speed of light cannot be
determined by the elastic properties of the ether, that is,
independently of the speed of the light source, and therefore the only
reasonable conclusion is: In so far as the speed of light is concerned,
Newton's particle model of light (rediscovered by Einstein himself) is
correct. The speed of light is c'=c+v, where c is the relative speed of
the photons and their source and v is the relative speed of the source
and the observer.

Pentcho Valev