Poussin, Nicolas: The Triumph of David (c1630)

The camera's eye is indiscriminate. Some accidental element will often slip in, the odd-thing- out which disturbs and disrupts the studium, and consequently fascinates. A strangely fancy collar, an elaborate shoe, a conspicuous body part, bad teeth, a large dirty hand, an awkward gesture " those are some of Barthes' examples. Whatever it is, the photographer wasn't looking for it, perhaps didn't notice it, but couldn't exclude it. It becomes the transfixing point, the sore spot, in the scene. 'A photograph's punctum is that accident which pricks me, but also bruises me, is poignant to me,' Barthes says.

And could a painting have a punctum? A puzzler. Details can't accidentally slip into paintings as they can into a photo. Yet a painting can have aspects the artist didn't notice. Or features that don't fit into its overall scheme. What's more, an artist could deliberately arrange things to look inadvertent or incongruous. You can imagine a photographer, who knew his Barthes, keeping his eyes peeled for precisely such awkward incidents. Why not a painter too? Why not, even, the painter whose motto was 'I have neglected nothing'?

Nicolas Poussin's The Triumph of David shows the triumphal return of David to Jerusalem, having killed the giant Goliath with his sling. The parade comes home, passes though the picture. The young hero carries the enemy's head on a pole. The crowds point and cheer. The stage is all set. But what's this? In the bottom left hand corner of The Triumph of David, in an open space in the paved forum, sits an architectural fragment. Fallen from somewhere, half-broken, it plugs its gap like an island, with empty floor all round it. It is pointedly contrasted with the human action.

Notice how the figures all overlap each other at one point or another, making a single 'land mass'. The fragment is the only isolated element. Amid the civic celebrations it stands apart, a piece of ruin, alien and inert and unmoved. It is unobtrusive. But when you notice it lurking there, it doesn't look inadvertent. Poussin has placed it pointedly at, and parallel to, the front of the picture. On the other hand, it is a detail that refuses to settle into the scene and refuses to make sense. Once you notice it, it separates. You find you have a choice. You look either at it, or at the rest of the picture.

You might expect it to be a symbol, to mean something specific. Scholarship has no suggestions here. Whatever it is or means, it remains a fascinating blank, an unignorable foreign body. But still, Poussin is not neglectful. He usually likes to fill his pictures with connections, patterns, echoes. Perhaps there's one here. Try this. What else in the picture compares to the stone fragment? Obviously the piece of human ruin, the giant's severed head. And prominent on its brow is the bloody wound where he was struck down, which points to another piece of stone, crucial to the story though missing from the picture " David's sling-shot.

Missing, but not quite forgotten. One of the spectators in the crowd tells the news to a late arrival, and points out on his own brow how the fatal blow was struck: it got him right there. And then, a further echo: to the left of the trumpeters, there's another opening " an area of blank stone wall, and it's filled not with an object but with a chip. A piece of the surface has been dashed out. It bears a hole like the giant's brow. The picture holds a ricochet of associations, going from stone shot to flesh wound to stone wound.

And this chip is also the only evidence in the scene of architectural ruination, such as would make the background temple into the source of the foreground fragment. And actually, isn't the fragment's secondary crumble of stone like a piece of slingshot rock? Follow the links, and they add up to a message of doom. The punctum strikes back. The picture relates the felling of the enemy to the fall of the state, and locks catastrophe into the hour of triumph.

THE ARTIST

Nicolas Poussin (1594-1665) is known as the most cerebral of painters. Born in France, he operated mostly from Rome. He painted 'high' subjects " from the Bible, mythology, ancient history and epic poetry. These were images for private study, for an intellectual élite, not for church and state. But their patient deliberation can accumulate a massive force.