Michelson-Morley experiment (A.A. Michelson, E.W. Morley; 1887)
Probably the most popular null-experiment of all time, designed to check the presence of the proposed "lumeniferous aether" by which light waves were thought to propagate. As the Earth moves through this aether, a lightbeam fired into the Earth's direction of motion would lag behind one fired sideways, where no aether influenced would be exist. This difference could be detected along the use of an interferometer.
The experiment illustrated absolutely no aether shift whatsoever, where one must have been quite detectable. Therefore the aether concept was discredited as was the idea that one computed the velocity of light as being inserted vectorially to the velocity of the emitter.