= Sagnac rotating-ring experiment
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In 1913 <Georges Sagnac> mounted a lamp, <collimator>, <beam splitter>, four <mirrors> and <photographic plate> on a horizontal rotating platform. The <mirrors> defined a closed polygon; the two beams traversed it in opposite senses. A slight <beam splitter> adjustment produced visible <interference fringes>. A motor brought the platform to steady <rotation>, and switching the lamp exposed the plate. Comparing opposite senses of <rotation> revealed a reversing displacement. Changing the <beam splitter> adjustment supplied a further check against optical deformation. The report compared measured and calculated displacements. <Georges Sagnac> interpreted this as evidence for a <luminiferous aether>; the relativistic derivation below reproduces the delay without that hypothesis.<Regarding the Proof for the Existence of a Luminiferous Ether Using a Rotating Inteferometer Experiment by Georges Sagnac>[Georges Sagnac, 1913 second report, inspected in William Lonc's 2008 English translation, pp. 77–80, §§1–3: rotating apparatus, photographic procedure, reversal and deformation controls, comparison table, historical interpretation]{ref}
The readout measures a relative <wave phase>, not two stopwatch readings separated by an optical transit time. Reversal isolates a contribution odd in <angular velocity>: if a signed <interference fringe> position is $q(\Omega)=q_0+k\Omega$ to leading order, then $[q(\Omega)-q(-\Omega)]/2=k\Omega$ cancels the fixed offset $q_0$. Changes of alignment or <temperature> correlated with reversal can survive this subtraction. Thus the experimental inference concerns a reproducible rotating-loop <phase difference>, with controls for disturbances; it is not by itself a unique test of an underlying propagation hypothesis.
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