On our visit to Birmingham this weekend, we popped into the Birmingham Museum and Art Gallery to see the Turner exhibition. This was excellent, and all the better for the fact that we got in free as it was only 40 minutes from closing time. Mmm — free art...

The Turner paintings were wonderful, but what really caught my attention on the way in was what is rather charmingly called a 'sticking-out' picture by Patrick Hughes. Imagine three tetrahedrons stuck inside a picture frame so that they are sticking out. Now cut off the ends of the tetrahedrons, and paint all the surfaces with a depiction of corridors in an art gallery, with paintings on the wall. When you come up to the painting, you have a very strong visual illusion that the tetrahedrons are sticking into the painting, and that the whole thing is three dimensional. As you move around, you see further into the corridors, and see more or less of the paintings. It's like walking around a tiny gallery, and it's somewhat spooky.

When I first saw it, it couldn't for the life of me figure out how it was done. I stood there — like a kid delighted by a magic trick — trying to see how it worked, and bobbing around to make the image move. It wasn't until I moved close to the painting and looked at it from the side that I saw the 'sticking-out' bits. Then the illusion popped in and out as my brain argued with itself about whether the image was 'sticking-out' or 'sticking-in'. I wouldn't suggest viewing it with a hangover, though.