If you’ve ever seen the painting reproduced, if you’re on a journey specifically to see it, then the next thing that will surprise you is the size. It’s smaller than you imagined—smaller than seems possible. There is so much to fit in, after all. The authorized Tate Gallery reproduction of The Fairy-Feller’s Master-Stroke I bought after seeing it the first time was almost twice the size of the picture itself, and was as unsatisfying as a photograph of a meal would be to a hungry man.

The painting is not the reproduction. The thing itself, in its frame, has a magic—in the color, in the detail—that no photograph, no poster, no postcard, ever seems to begin to capture.

So you look at the painting, seeing every brushstroke. Every nuance of paint on the daisies.

And you can look at it for hours before you notice something else about the painting, something so big and strange and obvious you can’t understand why you didn’t see it at once, or why no one else has commented upon it.

It’s not finished.

Much of the bottom of the painting, where the color choices seem odd and washed out, is only outlined on the light brown of the undercoat that covers the canvas. The fawn-colored grass that pushes the eye up to the Feller himself is fawn because Dadd—who took many years to paint it—ran out of time. He gave it away before it was done.

And there’s one final thing you will know, without question, if you’ve seen that painting in the flesh, and it’s this: he knew what he was painting. He had seen it, through those crafty eyes. He had gone on the great journey, the grandest of grand tours, and this was what he was bringing back.

Those of us who write fantasies for a living know that we are doing it best when we tell the truth. There is something that people will respond to—the True Quill, a Texan writer I met once called it. My novel The Ocean at the End of the Lane includes a lady on a Sussex farm who is older than the universe, and a strange flapping creature from somewhere outside space and time who comes into our young protagonist’s life in the form of an evil nanny. None of it’s true, except it feels right. It feels honest.

Before his madness, before the murder of his father, before the ill-fated journey to France (he was arrested on a train, when he attacked a fellow passenger, on his way to Paris to kill the emperor), Dadd’s paintings are quite pretty, and perfectly ordinary: forgettable chocolate-box-cover concoctions of fairy scenes from Shakespeare. Nothing special or magical about them. Nothing that would make them last. Nothing true.

And then he went mad. Not just a little bit mad, but quite spectacularly mad; a murderous patricidal madness of demons and Egyptian gods. He spent the rest of his life locked up—first in Bedlam, later one of the first prisoners in Broadmoor—and, after a while, he began to paint, trading his paintings for favors. Gone were the chocolate-box fairies of Come Unto These Yellow Sands. Now there was an intensity to his paintings and drawings of fairy courts, of Bible scenes, of his fellow inmates (real or imaginary), that makes those we have such treasures. They were worked on with an intensity and single-mindedness that is, quite simply, scary.