Complex sentence: Rotating Michelson's experimental apparatus does not affect the diffraction pattern which represents the negative result of Michelson's experiment but both light rays are propagating towards the mirrors then are reflected back and propagate in the opposite directions which essentially cancels the test of the ether drift.
Split into simpler sentences: Rotating Michelson's experimental apparatus does not affect the diffraction pattern which represents the negative result of Michelson's experiment. Both light rays are propagating towards a mirror then are reflected back and propagate in the opposite directions which cancels the test of the ether drift.