Observational Derivation of Einstein's``Law of the Constancy of the   Velocity of Light in Vacuo"
  On the basis of Galilean invariance and the Doppler formula, combined with an observational condition, it is shown that the constancy of the velocity of light {\it in vacuo} can be derived, together with time-dilatation and Lorentz contraction. It is not necessary to take the constancy as a postulate. 
Claudia Pombo and Theo M. Nieuwenhuizen