In July 1897, the commissioners of the Northern Lighthouse Board decided that the Firth of Forth needed two new lights: one at Barns Ness, near Dunbar on the mainland, and one out on the Bass. The Bass had loomed as an unlit hazard for centuries. Ships entering the Forth had to thread between the rock and the May Island and the North Berwick coast, and on dirty nights they did so without help. The cost of the Bass light came to 8,087 pounds. The first beam went out across the firth on the evening of 1 November 1902. Sailors approaching Edinburgh could finally see the Bass before it saw them.

David Alan Stevenson (1854 to 1938) belonged to the third generation of the lighthouse-engineering dynasty founded by his grandfather Robert Stevenson, who had also built the original Isle of May lighthouse in 1816. Robert Louis Stevenson was his cousin, the literary defector who broke from the family trade to write Treasure Island and Kidnapped. David stayed with the engineering. He produced a 20-metre tower of the local sandstone-and-basalt rubble salvaged from the castle, with quarters for keepers, oil stores, and the optical apparatus the technology of the day required. The original mantle ran on incandescent gas, vaporised paraffin oil converted to bunsen gas, kept burning around the clock. Until automation in 1988 a keeper had to be present to maintain it.

The keepers had the gannets for company. More than 150,000 northern gannets nest on the Bass each summer in the world's largest colony of the species, and their droppings produce 152 tonnes of ammonia a year, equivalent, by one calculation, to ten million chickens. The smell is part of the place. Supply boats came from North Berwick and Dunbar in fair weather, and in foul weather the keepers waited. The station was de-manned in 1988 when the Northern Lighthouse Board switched to remote monitoring from headquarters in Edinburgh. The new lamp is an ML300 synchronised bifilament 20-watt electric, biform-mounted to give continuity if one filament fails. Twenty watts now does the work that once required vaporised paraffin and three keepers.

You cannot land at the lighthouse without permission from the owner, Hew Hamilton-Dalrymple, whose family has held the Bass since 1706. Boats from the Scottish Seabird Centre in North Berwick circle the rock through the season, weather permitting, and a smaller number land at the only beachable spot below the castle ruins. The lighthouse sits about midway up the rock's southern face, white against the cliff, the kind of object that catches the eye from twenty miles out at sea. From the mainland it is a fixed point in the seascape, a small bright marker against a dark volcanic plug, the modern function quietly outlasting all the older drama the rock has held.