Inside the Lantern, the Fastnet Lighthouse, Co. Cork

The first time I visited the Fastnet, I spent most of my time just looking. You do that on the rock. The granite, the sea hammering below, everything about the place demands it. But the moment I remember most clearly happened inside the lantern room itself.

There's a small hatch at the base of the lantern assembly. You climb up through it and look directly into the Fresnel lens from below. The geometry is extraordinary. Concentric glass prism rings, heavy brass framework, the whole mechanism designed over a century ago and still precise enough to make an engineer weep. I was composing a photograph of the interior when I noticed something. As the lantern rotated, a brief rainbow would flash across the vertical support post. A tenth of a second. Then gone. Then back on the next rotation.

I wanted it in the frame. Most exposures missed entirely. A couple caught a faint smear of colour but nothing convincing. Eventually one frame caught the full spectrum, violet through red, refracted cleanly across that metal post by glass prisms designed to throw light twenty miles out to sea. Not to make rainbows for photographers crouched underneath!

That's what I love about the Fastnet. An early twentieth century industrial masterpiece that still surprises you.