In Fizeau’s apparatus, a beam of light shone between the teeth of arapidly rotating toothed wheel, so the “lantern” was constantlybeing covered and uncovered. Instead of a second lantern far away, Fizeausimply had a mirror, reflecting the beam back, where it passed between the teeth of the wheel. The idea was, the blip of light that wentout through one gap between teeth would only make it back through the same gapif the teeth had not had time to move over significantly during the round triptime to the far away mirror. It was not difficult to make a wheel with ahundred teeth, and to rotate it hundreds of times a second, so the time for atooth to move over could be arranged to be a fraction of one ten thousandth ofa second. The method worked.


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