EXHIBITIONS / Imitation: the sincerest form of flattery: Why include another artist's work in your own painting? A new show at the National looks into a peculiar kind of homage

'PICTURES in Pictures', at the National Gallery, is a fascinating exhibition and many will wish to see it more than once. The atmosphere, though, is marred by some obtrusive displays, over-emphatic explanatory labels and a huge photograph of one of the paintings. If we have the picture, what need of the photograph? One of the themes is thoughtfulness, and surely the works on display need a more meditative environment.

Thoughtfulness about painting and its relation with learning is a metaphor for the National Gallery's role. All 26 paintings in the Sunley Room (where admission is free) come from the gallery's own collection, and they have one thing in common: a representation of another picture or sculpture. The earliest works are from the 15th century and the latest is Degas' 1886 portrait of Helene Fouart. The circumstances and motives for quoting must have changed over the centuries, but the underlying impulse usually seems to be respectful. There is no mockery of other artists, except perhaps in Hogarth. The interior pictures are mostly in the later work, as a form of homage.

Especially, for instance, in the study that was once given to the Old Masters. Wallerant Vaillant, the Franco-Dutch 17th-century painter and mezzotinter, shows a picture of a boy drawing in front of a cast of the Christ Child by Michelangelo. At his feet is an open folio of drawings, and we guess that he hopes to emulate their example. It is an allegory of academic art. Perhaps the canvas is doubly academic, for it looks suspiciously like a copy. Not a consummate picture, it is the sort of work great galleries keep in their reserve collections. In its present context, however, it is suddenly of interest, as part of a tradition that gave inspiration to so many artists.

One of the oldest pre-academic stories about painting concerns St Luke, who is said to have taken the likeness of Christ (this is a 10th-century Greek myth) and for that reason is the patron saint of painters. A follower of Quentin Massys represents Luke at work on a picture of the Virgin and Child. This is a tall, thin painting because it's the side shutter of an altarpiece; and see how the saint's easel would have balanced and mirrored the crucifix in the central part of the triptych. Luke has an easel because the picture is really a description of an early 16th-century painter's studio. One wonders whether the person who painted this hasn't introduced his self-portrait as that of the saint whose craft he followed.

Two more northern European paintings, of a later date, show how the themes of the artist's studio never became totally secularised. Here's a rare picture by Gabriel Metsu, unusual because it shows a woman drawing. The domestic scene has a devotional atmosphere, all the more so because the female artist is drawing from an engraving of Gerard Seghers's Christ at the Column. I wonder why this painting is not better known. There are 2,000 paintings and more in the National Gallery, and some are bound to elude our attention. I doubt many can say that they know Quiringh van Brekelenkan's A Tailor's Shop. But it's a good picture and has a nice place in this exhibition because this very ordinary Dutch artisan has a still-life painting in his place of work.

It almost has a religious flavour because the picture within a picture is like an icon rather than a bit of decorative furniture. Also the shop can be considered a studio of sorts. The studio as a place of mystical as well as practical endeavour is a theme that artists love. Cezanne exemplifies it here. His early studio picture shows the back of one of his canvases and his old stove - mundane subject matter that gave rise to a quite thrilling painting. The canvas that we can't see grows ever more mysterious, while it is also alive with potential. Cezanne's picture announced the modern theme of the metaphysical studio interior whose greatest exponent would be his follower, Matisse.

Matisse isn't in the show. I mention him because he was so assiduous a painter of pictures within pictures. Notably, the pictures he quoted were always his own. It seems that earlier artists were not always so engrossed in their personal vision. References to other works usually imply that greater things exist than those of which they are themselves capable. Initially, such respect is given to Christianity, as in Hendrick van Steenwyck's Interior of a Gothic Church, Petrus Christus's Portrait of a Young Man, Bellini's Blood of the Redeemer or the Presentation by the Master of the Life of the Virgin. With the Renaissance, though, and especially in Italy, quotation becomes classical, pictures within pictures speak of more than one god, and brooding over everything is the thought of human death.

An interesting bit of hanging is to put Parmigianino's Portrait of a Man next to Lorenzo Lotto's Lady with Drawing of Lucretia. The first appears darker than usual, the second lighter. They are united by their baleful and threatening nature. The standard theory about the Parmigianino is that it represents a collector. If so he has gained his treasures by murder. He holds a casket, various objects are on a table before him and behind is a classical relief showing Mars and

Venus with Cupid. The reference to classical culture underscores the painting's menace: it is as though gentle Christianity could have no effect on the order of art. Similarly with the loot. It is a wedding portrait, so should be happy and optimistic: but its subject points to a drawing of a woman who killed herself after being raped.

The theme of sexual admonition also turns up in Poussin, whose Cephalus and Aurora depicts Cephalus turning from the goddess to look at a portrait of his wife; and of course in Hogarth, whose Marriage a la Mode - a sequence of six paintings - has been put in a separate section. This is appropriate for both aesthetic and human reasons. Hogarth is the least subtle of all the painters in the exhibition and the most tedious in his moralising. I think he was motivated by envy as much as by the ethical sense for which he is so often applauded. He envied not only the rich but also, worse, better painters than himself. Marriage a la Mode is stuffed full with other people's art, and it seems obvious that he loathes all of it. No other painter in this exhibition shows such embittered disdain for other artists' creations.

Some of them, though, use the picture within a picture to insist on their own personalities. Murillo's self-portrait is a marvel of the artist's power. We see his portrait in an oval frame like a mirror, this looking glass standing on a shelf on which are placed the attributes of his profession: paper, pencil, palette, brush. But his hand comes out of the oval in a trompe-l'oeil manner, as though Murillo had come to life through being painted by himself. For the picture is also about death. A Latin inscription, just like a funerary tablet, dedicates the work to his children, the artist has contrived to make himself look neither young nor old (52, let us say), the atmosphere is dark and the composition that of a frontispiece of a book: promising much in future pages, but still in commemoration of a departed person.

Not even Velasquez or Picasso played out such Spanish games with death and pictorial life. Velasquez's Kitchen Scene was painted when he was a young man and is not totally assured. A puzzle about it is whether its top right-hand rectangle is a depiction of a painting of Christ in the House of Mary and Martha or an aperture in the kitchen wall (or a mirror?) through which these two humble women, preparing a meal, might discern a Biblical revelation. If only they had not other work to do . . . So I put this mysterious picture next to the van Brekelenkan, as examples of pictures within pictures suggesting that there is more to life than work: death, certainly, but art too when you can get it in life.

This I hazard is the message of the most perfectly beautiful of all the paintings, Vermeer's A Young Woman standing at a Virginal; not the one next to it of the same title and date (about 1670) with a picture of a procuress in the background but the clearer of the two in which we find a painting of Cupid. Someone ought to write a book about this little fellow as he appears in art. A chubby and senseless killer, often as not, all too ready to do his mistress's mad biddings. In this picture he looks more like Apollo, god of art and learning. That's good: the woman in Vermeer's painting was about to get married. The mood of this Vermeer is unique to the picture itself. But it hangs well with the Degas. The old misogynist felt very tender towards Helene Rouart, the daughter of his old friend and patron. She is shown in her father's picture-hung study. Did Degas place her there because he didn't wish her to marry?

Continues at the National Gallery, WC2 (071-839 3321), until 19 September.

(Photographs omitted)