Designed by the American physicist, Albert A. Michelson, this type of interferometer measures the speed of light along two different paths. Using an instrument of this sort in 1887, Michelson and Edward Morley showed that the speed of light is not affected by the motion of the earth through the supposed luminiferous ether. This example was used with the Mendenhall gravity apparatus at the U.S. Coast & Geodetic Survey. The wooden box is marked "INTER F.B." and "C.&G.S."
Ref: Clarence H. Swick, “The Force of Gravity and Methods of Measuring It,” Bulletin of the American Geographical Society 44 (1912): 183-189.
Clarence H. Swick, Modern Methods for Measuring the Intensity of Gravity (Washington, D.C., 1921), fig. 16.
W. H. Burger, “The Measurement of the Flexure of Pendulum Supports with the Interferometer,” Appendix 6 in Report of the Superintendent of the Coast & Geodetic Survey Showing the Progress of the Work from July 1, 1909 to June 30, 1910.
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