Maxwell theory of propagation and the ether
The fact that light travels at a finite speed was first discovered in 1676 by danish astronomer Ole Christensen Roemer.
A proper theory of the propagation of light didn’t come until 1865, when the British physicist James Clerk Maxwell succeeded in unifying the partial theories that up to then had been used to describe the forces of electricity and magnetism.
Albert Michelson and Edward Morley in 1887 compared the speed of light in the direction of the earth’s motion with that at right angles to the earth’s motion. To their great surprise, they found they were exactly the same!