Complex sentence: Michelson is testing for the existence of a ether drift produced, at a point on the surface of the earth, by the earth's motion through a stationary ether where the incident light beam is split into two light rays.
Split into simpler sentences: Michelson is testing for the existence of a ether drift produced, at a point on the surface of the earth, by the earth's motion through a stationary ether. Michelson's incident light beam is split into two light rays.