All the details here. Note the picture of the shack on Lookout.

"One of the most important scientific experiments of the 1920s took place on Lookout Mountain, a 6,812-foot high point on the south ridge of Mount Baldy. [Lookout Mtn. is a spur of Mount Baldy also known as Mount San Antonio; Mt. San Antonio is the highest mountain (10,064') in the San Gabriel Range - ed.]. This was the experiment by Albert A. Michelson [...] to determine the speed of light. In 1922, Dr. Michelson set up a rotating octagonal prism on Mount Wilson, called Station Michelson. On Lookout Mountain, 22 miles [35.5 km - ed.] away, he installed a concave mirror two feet in diameter, called Station Antonio. He then shot a beam of light [from] Station Michelson to the mirror at Station Antonio; the light beam was reflected back to Mount Wilson and the precise time measured. To insure the absolute accuracy of the experiment, it was vital to know the exact distance between the two stations. The distance was measured by the U.S. Coast and Geodetic Survey by triangulation from a 40-kilometer surveyed base line in the valley to the south. To quote William Bowie of the U.S.C.G.S., It is believed that the length of this line has been determined with greater accuracy than that of any line of triangulation in this or any other country. The possible error in the 22-mile distance between Mt. Wilson and Lookout Mountain was less than one-fourth of an inch! [...] Today, three inline concrete piers, the tallest one 42 inches high with a metal tablet marked Antonio 1922, stand alone amid the brush on Lookout Mountain."
Verum audaces non gerunt indusia alba. - Ipsi dixit MCMLXXII