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The ether drift experiment as performed by Kennedy with a reduced optical system in helium has been repeated with the same apparatus somewhat modified and the same results obtained. The interferometer has been improved by resilvering the mirrors so that 1/1500 of a fringe shift could be detected by an observer with good eyes, and 1/500 by an observer with poorer eyes. Additional readings, which eliminate steady thermal shifts of the fringes, have been made and these show no ether drift to an accuracy of about one kilometer per second.
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K. K. Illingworth (1927) studied this question.
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