All evidence, both theoretical and experimental, spoke in favour of the concept that speed of light (relative to the observer) does vary with the speed of the observer, and yet Einstein wrestled with this concept "over a lengthy period of time, to the point of despair". Eventually he procrusteanized time in order to create a world where the false antithesis (the speed of light does NOT vary with the speed of the observer) is ostensibly correct:
http://www.aip.org/history/einstein/essay-einstein-relativity.htm John Stachel: "But here he ran into the most blatant-seeming contradiction, which I mentioned earlier when first discussing the two principles. As noted then, the Maxwell-Lorentz equations imply that there exists (at least) one inertial frame in which the speed of light is a constant regardless of the motion of the light source. Einstein's version of the relativity principle (minus the ether) requires that, if this is true for one inertial frame, it must be true for all inertial frames. But this seems to be nonsense. How can it happen that the speed of light relative to an observer cannot be increased or decreased if that observer moves towards or away from a light beam? Einstein states that he wrestled with this problem over a lengthy period of time, to the point of despair. We have no details of this struggle, unfortunately. Finally, after a day spent wrestling once more with the problem in the company of his friend and patent office colleague Michele Besso, the only person thanked in the 1905 SRT paper, there came a moment of crucial insight. In all of his struggles with the emission theory as well as with Lorentz's theory, he had been assuming that the ordinary Newtonian law of addition of velocities was unproblematic. It is this law of addition of velocities that allows one to "prove" that, if the velocity of light is constant with respect to one inertial frame, it cannot be constant with respect to any other inertial frame moving with respect to the first. It suddenly dawned on Einstein that this "obvious" law was based on certain assumptions about the nature of time..."
Yet the hoax was not fully successful - there is an Achilles heel to Einstein's relativity:
A light source emits a series of pulses the distance between which is d (e.g. d=300000km). A stationary observer/receiver measures the frequency of the pulses to be f=c/d: