Prior to 1887, St. Agnes Lighthouse safeguarded vessels in the southern approaches to the Scilly Isles and Bishop Rock Lighthouse did the same to the west. But the unlighted dangerous northern limits were hazardous, especially in fog. Accordingly, a lighthouse was built on Round Island, a small granite islet six miles north east of Bishop Rock and two and a half miles north of St. Mary’s.
The top of Round Island forms a platform on which Trinity House built a lighthouse and dwellings in 1887 in extremely difficult conditions; the sheer rock face made the unloading of building materials almost impossible. The work was carried out by William Tregarthen Douglass of the famous Douglass lighthouse-building dynasty. Today the only access, apart from by helicopter, is by a flight of steps cut into the solid rock.
An enormous Chance Brothers biform hyperradial optic—similar to that installed at Bishop Rock in the previous year—was installed in 1888, arranged to give single red flashes at equal intervals of 30 seconds, thereby distinguishing it from the white double flashes of nearby Bishop Rock.
This optic was replaced in 1967, and once again when the lighthouse was automated in 1987.
The lighthouse is now monitored and controlled from Trinity House’s Planning Centre in Harwich, Essex.