I visited the Pre-Raphaelite room at the Tate first in my early twenties: in my teens I had loved the work of comic book artist Barry Windsor-Smith. He made no secret of his Pre-Raphaelite influences, and I wanted to see them close-up—Millais and Waterhouse and the rest. I went there, and I liked the paintings, and admired them, and decided that I did not like the work of Dante Gabriel Rossetti as much as I rather suspected Dante Gabriel Rossetti had, and the Burne-Jones picture of the ladies going downstairs made me catch my breath.

They had several Dadd paintings too, there almost by default, as if there was nowhere else to put them. I saw The Fairy-Feller’s Master-Stroke, and I was obsessed.

The year before I had received a copy of a book to review, of photographs mostly taken by a Victorian doctor named Diamond, of the inmates of Bedlam. Hopeless bedraggled lunatics who wring their hands as they squint at the camera, posing awkwardly for the period of time it took for the photographs to be exposed; their faces are frozen, although their hands often blur into things like the wings of doves. Portraits in madness and pain, and in only one of the photographs in the book was a man, a lunatic like the others, actually doing something.

The madman in the photograph, which was taken by Henry Hering in 1856, has a beard. He has an easel in front of him, on which he is executing an oval painting of remarkable intricacy. He stares craftily at the camera, and there is a small, fierce smile on his face. His eyes glitter. He looks squat and proud, and when, a year later, I saw, for the first time in the flesh, his masterpiece, The Fairy-Feller’s Master-Stroke, the first thing I realized was that the white-bearded sorrowful dwarf who dominates the center of the painting, staring out at the watcher, is Richard Dadd grown old.

The visitors to the Tate Gallery who visit the Pre-Raphaelite rooms are there for their own reasons, and are responding to something distant and melodic. The Waterhouses and the Millaises and the Burne-Joneses exert their own magic: spectators wander past the paintings, their lives enriched and made special. The Dadd, on the other hand, is a snare, and those people with a place in their soul for it—and I am one of them—are hooked. We can stand in front of that painting for, literally, hours, lost in it, puzzling over these fairies and goblins and men and women, trying to understand their size, their shape, their eccentricities. Every time you look at it you discover something, someone, you have not seen before.

Dadd knew who they were, the people in the painting. He knew their lives. He knew what they were. You know that when you see them. He wrote a poem about them, in Broadmoor, in 1865, called “Elimination of a Picture & Its Subject—Called the Feller’s Master Stroke.” That’s how we know the title. He was a better painter than he ever was a poet.