The light from a coastal lighthouse needs to be visible from great distances and be bright enough to penetrate bad weather. This lighthouse lens concentrates the light of the lamp into three beams which sweep the horizon as the lens rotates. This gives a characteristic pattern of flashes to enable sailors to identify the light.

The Eilean Glas optic is based on the dioptric or Fresnel principle, in which the light is focused and directed try concentric rings of prisms. This design allows the construction of lenses of great power and size that are not excessively thick. Nevertheless, in this light, the glass elements alone weigh over 3 tons and were made by Chance Brothers of Birmingham, the world leaders in this field at the time.

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. During this period the engineering and design of the Scottish lighthouses was managed by four generations of the Stevenson family, an extraordinary dynasty of engineers which also included the writer Robert Louis Stevenson.

This optic is part of an outstanding collection of lighthouse equipment which is held by the National Museums of Scotland.

Inv. L2000-4405