Lighthouse optic from Eilean Glas lighthouse, 1907.
© Science Museum / Science & Society Picture Library
Optic taken from the Eilean Glas Lighthouse, Scalpay Island, Scotland, on display in the Science Museum, 2007. The Eilean Glas optic is based on the dioptric or Fresnel principle, in which the light is focused and directed by concentric rings of prisms. T