ART / Mirror, mirror on the wall: Tom Lubbock on the jokes, references and commentaries hidden in Pictures in Pictures at the National Gallery, London

ST LUKE is supposed to have been a painter, and there are some old icons of the Virgin Mary that are supposed to have been painted by him - from life, as it were. This is all legend. But on the basis of it, St Luke himself gets painted quite a lot, shown at work before his easel. The National Gallery has one of these pictures: St Luke Painting the Virgin and Child, by a follower of Quentyn Massis. The sitters aren't visible, and since the Saint here is a grown man, he's presumably painting the scene from imagination, or, at a pinch, memory. But perhaps not. It wouldn't be that surprising to find the baby Jesus himself present. Artists of the early 16th century weren't much troubled by anachronisms. After all, the unknown artist here portrays Luke as a contemporary painter in a contemporary studio. And as for what Luke is painting - the easel is set obliquely, but you can see clearly enough - it's not a primitive icon, but a picture pretty much in this artist's own manner.

This is one of 26 pictures from the National's collection now grouped together in a thematic show called 'Pictures in Pictures' - paintings with paintings depicted within them, from the 15th century up to Degas and Cezanne. It's a big theme, with potentially thousands of instances, and like any case of self-reflexiveness in art - plays within plays, novels within novels - it's guaranteed to raise lots of questions (about the nature of painting itself, for example). But the first thing you notice is the simple technical challenge of the device. How to paint a painting, and someone else's painting at that, and maybe at a sharp angle, too?

There are ways, naturally. Blurring the image within the image, or darkening it, or making it faint, or painting it in simple broad strokes usually does the trick. In Cezanne's Stove in the Studio there's a bright square smudge on the back wall - one of his own paintings, one guesses, now translated into the vaguest impression of itself. But you see the business most clearly when things go wrong. Cognoscenti in a Room Hung with Pictures, by an unknown Flemish artist of the 17th century, was never going to be a very interesting picture, but it could have been virtuosic at least. It represents a chamber with 40 paintings lining the walls from floor to ceiling, all sorts of genres and styles, and some copied from identifiable originals.

But the artist has little success in distinguishing the look of the pictures-within from the rest of his picture, or in indicating a range of different styles. The paint register, the sense of detail, is almost uniform. The painted objects are almost as real as the real ones. It is, fatally, all his own work. And with the pictures on the side wall, which are seen at a very sharp angle, the artist can't resist turning the images slightly more front-on than they should be. As the exhibition leaflet aptly says, these are 'not paintings in a painting, but paintings on a painting'.

But what counts as failure in one picture can be turned to advantage in another. The ambiguous status of the picture-in-the-picture lends itself to trickery, a touch of Magritte. There's a Self-Portrait by Murillo in this vein. In a way it's an absolutely straight self-portrait, except that the artist has painted an internal oval frame around his image, within the edges of the canvas, so as to make a painting-within. But here's the trick: the artist's hand pokes out of this frame; the image suddenly gets real. Such games with the pictorial illusion, though they always seem somehow 'deep', exploit the device in a rather blank and pointless way. (Actually, I think Murillo is more boasting about how persuasive his powers of illusion are, rather than emphasising their artificiality.)

Hogarth finds more of a point for them in the moral comedy of his Marriage a la Mode. In these six paintings the picture-within is used in every possible way: to indicate the taste of the protagonists, to add allegorical commentary to the action, to satirise the work of other artists etc. Hogarth likes to pack it all in. But the best joke is when the picture-within responds to what's going on in the room, becoming a proxy audience. In the first episode, a painted Gorgon's head looks on aghast at the unsuitable marriage being contracted before its eyes. In a later one, it is St Luke himself, depicted with pad and pencil, who gapes out of his frame and begins to sketch the husband's murder.

Velazquez's Kitchen Scene with Christ in the House of Martha and Mary, on the other hand, is a puzzle, and partly because you really can't tell what's what. The front of the picture shows one of the artist's beautiful low still- lifes, fish and eggs on a table, with a contemporary kitchen girl pulping garlic in a brass mortar. All palpable, mortal reality. But the square New Testament scene behind her - is it a painting on the wall, or is it a view through a hatch into another room? There are indications both ways. In favour of it being a view: the perspective of the Biblical scene is roughly continuous with the main space; as a picture, it would be an inelegant and lopsided composition; this section of the wall is in shadow, yet the scene is clear. But then, in favour of it being a picture: the Biblical figures are painted in simple, crude strokes, which suggest a painting painted rather than (say) distance painted; the light falls from the opposite direction to the light in the kitchen. It appears in fact to be deliberately ambiguous, but this only leaves another mystery. Why? The picture is clearly saying something about housework (the still-life glorifies it, the Bible story says no, turn to the Lord). But whether the scene is view, picture, or both, seems to make no obvious difference to its meaning at all.

It's in Vermeer, in his Woman Standing at a Virginal, that the device is shown to fullest, though most unforced, effect. It's used principally to introduce another dimension of reality. This is something that religious and mythological pictures often do in a literal way: natural and supernatural beings co-exist in the same space, and a real embodied Cupid can appear on the scene to hover over the enamoured. But Vermeer's is a secular, realist art, and this is a bourgeois interior seen in everyday light. The conventions disallow direct interventions from other worlds - except by way of the picture in the picture. The main one here shows a Cupid, and there's allegory at work, that's clear enough. Holding up a single card, the Cupid perhaps symbolises the woman's fidelity to one person. It's also, incidentally, the nearest thing to - what is very hard to imagine - a picture of a naked body by Vermeer. Still, it's not quite by Vermeer. He paints it so as to indicate the work of another hand (it may be a copy after a little-known artist, Cesar van Everdingen). And at all events, unlike in the Velazquez, it's clearly a picture.

Yet it's something more. Vermeer intimates that the Cupid has its own reality, too. It looks half-alive. It's lit from the same direction as the room. The contrast in paint-register is not that sharp. And the relationship between him and the woman is more than purely allegorical. They share not quite a physical space, but certainly a psychic one. Each seems to know the other is there. The woman looks out of the picture, catching our eye; the Cupid waves. Her head falls within his frame. The two are of the same mind. You could say he is her thought-bubble - or, on the other hand, her guardian angel. The point is, by using this device, Vermeer doesn't have to choose between a psychological and a supernatural statement. And there are no jokes or tricks about it. The rules of realism are scrupulously, serenely maintained. It's just a woman standing in front of a picture on a wall. It's just a perfect coincidence of painting and life.

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(Photograph omitted)