Lighthouse optic from Eilean Glas lighthouse, 1907.
© Science Museum / Science & Society Picture Library
Optic taken from the Eilean Glas Lighthouse, Scalpay Island, Scotland. 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. This design allows the construction of lenses of great power and size that are not excessively thick. Nevertheless, in this optic, the glass elements, made by Chance Brothers, Birmingham, weigh over 3 tons (3000 kg). The first light on the site was built in the 1780s to help mark the Minch - the channel between the Isle of Lewis and the Scottish mainland - and was updated in 1907 with the optic shown here.