My photo shows part of the six-sided fresnel lens in South Stack Lighthouse, on Ynys Lawd a small island off Anglesey, Wales.
It was an interesting tour/visit. As I understand it, that lens assembly was constructed in 1873 by Chance Brothers of Birmingham, and is still in use today. What is further fascinating is that the lens assembly 'floats' on a trough of liquid Mercury for an almost frictionless rotation.