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Edward W. Morley and Albert A. Michelson tested whether the orientation of an interferometer revealed motion through the proposed luminiferous aether. Perpendicular optical paths were recombined to form interference fringes; rotating the instrument tested for the predicted change in their relative travel times. The measured shift was much smaller than the prediction based on a stationary luminiferous aether and the Earth's orbital motion. The experiment constrained that model through a differential optical measurement, rather than directly timing a single light pulse.^[ref]

An interferometer compares round-trip light travel times along perpendicular arms as its orientation changes. The absence of the classical stationary-aether fringe shift at the expected scale constrains orientation dependence of the round-trip speed of light. The result is consistent with special relativity, but alone does not select it uniquely from every alternative theory.^[ref]

The instrument's stone support and floating mounting allowed the experimenters to change its orientation while limiting mechanical disturbance. The measured quantity was the fringe shift produced as the apparatus rotated, rather than a direct timing of a single light pulse.^[ref]

The original report by Albert Abraham Michelson and Edward W. Morley describes an interferometer experiment constraining the orientation dependence of light propagation. Scanned report: history.aip.org/exhibits/gap/PDF/michelson.pdf