The enclosure for the launch/return equipment of the Michelson speed of light experiment. (photo courtesy of the Huntington Library and the Carnegie Observatories.).
Mount Wilson served as the launch site for a brilliant beam of light that shone across the San Gabriels, bounced from a return mirror located on a side ridge of Mount San Antonio, and re-traversed the 22 miles separating the two mountains. A rapidly rotating mirror, driven by compressed air, screamed like a siren. By measuring the tiny change in angle of the mirror during a round trip of a burst of light, taking only two ten-thousandths of a second, the speed of light was measured.
This elegant experiment, devised and carried out during 1924-26 by Albert A. Michelson, America’s first Nobel Prize winner in a science field, provided the first modern and truly accurate measurement of this fundamental physical constant. Not only did Michelson have to make the very challenging measure of the light travel time, but the distance from Mount Wilson to the retro-reflector site had to be surveyed with high accuracy.
The housing for Michelson's apparatus is now gone, but a plaque on the support structure and pier memorializes this milestone experiment in twentieth-century physics.
This plaque on the enclosure pier commemorates the Michelson experiment.