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.
Michelson is testing for the existence of a ether drift. This ether drift is produced by the earth's motion through a stationary ether where the incident light beam is split into two light rays. This was at a point on the surface of the earth.