The Fresnel Lens in the Point Cabrillo Light House in Mendocino, California. Taken during a rare tour of the lens. Image was taken while the lens was rotating. They obviously had just finished polishing the glass and brasswork. Amazing technology which is still working daily from around 1890.
The current assembly rotates on a large (approx. 6") ball bearing. The original bearing system was a pool of Mercury of which the lens assembly "floated" on, thus requiring the huge lens to be balanced.