The experiment of Michelson and Morley

Light was assumed to propagate in a medium called ether which was all pervading and stationary. Earth’s motion through ether was expected to have an effect on the velocity of light in the direction of the relative motion by the law of addition of velocities. Michelson–Morley experiment used an interferometric technique to detect this effect and got a null result, which was the first strong evidence against existence of ether and eventually had a natural explanation in special theory of relativity.