English: Apparatus used in physicist Albert A. Michelson, Fred Pease and astronomer Francis Pearson's 1930-35 determination of the speed of light. It consists of a mile long 3 ft diameter vacuum chamber in a Southern California valley containing an optical system with two large concave mirrors at either end. Inside the vacuum chamber a beam of light from an arc lamp is reflected from an eight-sided mirror spinning at 512 revolutions per second, then makes ten passes through the tube, after which it returns and reflects again from the same face of the mirror. During the light beam's ten-mile journey the mirror rotates through a small angle, so the reflected beam has a small angle to the outgoing beam. The apparatus measures this angle, which is proportional to the time of flight of the beam. The tube is evacuated to a pressure of about 10 Torr. E. C. Nichols designed the optics.

Michelson died in 1931 with only 36 of the 233 measurement series completed, but Pease and Pearson carried on. The experiment's accuracy was limited by geological instability and condensation problems, but in 1935 a result of 299,774 ± 11 km/s was obtained, the most accurate measurement of the speed of light to that date.