Hippolyte Fizeau tested the effect of moving water on the propagation of light. Two beams traversed flowing water in opposite senses and were recombined so that a travel-time difference appeared as a shift of interference fringes. Reversing the flow supplied a controlled comparison. His 1851 report compared the observed shift with competing accounts of how motion of a transparent medium affects light, making a small propagation change measurable without directly timing the transit.[ref]
The Hypotheses Relating to the Luminous Aether by Hippolyte Fizeau. 1851 report, opposite propagation through moving water, flow reversal and fringe shifts
A physicist.[ref]
https://www.academie-sciences.fr/pdf/dossiers/Fizeau/Fizeau_od1.htm. Independent biographical identity and professional specialization
Portrait of Hippolyte Fizeau
. Charles Reutlinger; Académie des Sciences / Smithsonian Institution Libraries, SIL14-F002-10. Commons PD-old/life-plus-70 claim in country of origin, with a separate-US-tag request. Longer-term jurisdictions may differ., via Wikimedia Commons. Source. Articles by others on the same topic
Hippolyte Fizeau (1819–1896) was a French physicist known for his significant contributions to the field of optics and for his work on the speed of light. One of his most notable achievements was the first successful measurement of the speed of light in 1849, using a rotating toothed wheel apparatus.