Vladimir
c was just a constant, and he explained it as a ray of light. It was not observational light, just a ray. Later it was lightening, which is seriously difficult to see with! There was no observation in Einstein. You find me an example. What he said he was doing, and thought he was doing, is irrelevant, it is what he did which matters.
There is no relativity in physical existence. The 'relativity', or more precisely, variance in timing, is in the receipt of light, which although existent in its own right, is a representation of the reality which occurred. And that occurred in a definitive discrete physically existent state.
Paul