A precious stone set in a silver sea: The Wilton Diptych: Andrew Graham-Dixon deciphers the royal message for so long concealed within medieval England's most famous painting

ONE OF the most remarkable things painting can do is take you instantly elsewhere, plunge you into an alien and distant world. 'Making and Meaning: the Wilton Diptych', at the National Gallery, a thoroughly engrossing exhibition devoted to the most enigmatic painting to survive from the English middle ages, is a vivid demonstration of the transporting capacities of art. It is also an object lesson in just how much - how much emotion, belief, aspiration and failure - can be contained on no more than a couple of tablets of painted wood.

The Wilton Diptych has been a mystery for centuries. The painting's blend of naturalism and otherworldliness is both subtle and slightly puzzling: its most famous figures, those faintly insouciant long-necked angels with the most famous angels' wings in art, are a strange and compelling hybrid of real people and real birds, supernatural beings formed from a blend of observation and imagination. But they are just one part of an intricate symbolic scheme that has never been satisfactorily deciphered.

What, exactly, is the nature of the encounter which the picture depicts? What is the relationship between the kneeling King Richard II, flanked by three saints in a wild and wooded landscape, and the Madonna and Child in a paradise garden crowded with angels? Some momentous event appears to be taking place: that much is clear from the busy gesticulation of four of the angels, who are pointing at the figure of Richard; and also from the strange, open-handed gesture of the king, who appears to be waiting for some sign (approval? benediction?) from the Virgin and Child. Something is happening, some kind of transaction that bridges the two abutting hinged panels, worldly and heavenly, of which the diptych is made - but just what that might be has escaped us, separated as we are from the picture by a distance of six centuries.

The National Gallery exhibition (and its superb catalogue) may, however, mark a watershed in the history of the painting's interpretation. Recent cleaning of the picture has revealed a tiny detail, no more than a couple of centimetres across and previously obscured by dirt. The stave of the red and white banner held by the angel on the extreme left of the right-hand panel is crowned by a minute orb, previously thought to be blank. Restoration has revealed a tiny picture within the circumference of this orb: the image of a green island with trees on its horizon and a turreted white castle at its centre, floating in a dark sea which - according to the National Gallery's team of restorers - was originally painted silver but is now permanently tarnished.

This image, discovered in the greatest surviving painting of Richard II, inevitably calls to mind lines in the greatest piece of literature about him: John of Gaunt's description of England, in Shakespeare's Richard II, as 'this little world, / This precious stone set in a silver sea'. This suggests that Shakespeare may well have known the Wilton Diptych, or at least an image of an island very like the one that has been found in the picture. It may also provide the key to unlock the meaning of the painting.

The banner in the picture has until now been regarded as a conventional symbol of Christ's Resurrection. How this might have related to Richard II has never seemed entirely clear. But if the banner was meant, as now seems almost certain, to symbolise 'this realm, this England', the red and white flag of St George rather than of the risen Christ, then the picture becomes suddenly more intelligible. It becomes a subtle piece of royal self-invention: an assertion, by Richard, of his divinely sanctioned right to rule. The king, kneeling and empty handed, has given his realm into the power of the Virgin in the form of the banner. She has accepted it, and it is about to be passed back to the king by the attending angel (a fact sign- posted by that angel's pointing finger, directed at Richard). The King will rule England under the protection and with the blessing of the Virgin.

The Wilton Diptych seems to have been painted around 1397, near the end of Richard's troubled reign, but the symbolic link which it proclaims between the King and the blessed Virgin (the picture has the quality of a mystic marriage, a betrothal in which Richard presents the Queen of Heaven with the dowry of his kingdom) may go back some 16 years before that to one of the most dramatic incidents in English history. The painting may refer to Richard's (and the Virgin Mary's) involvement in the Peasants' Revolt.

In 1381, more than 40,000 peasants marched on London under the leadership of Wat Tyler to protest against the Statute of Labourers, which fixed maximum wages during the labour shortage that followed the Black Death. Before negotiating with them, according to Sir John Froissart's contemporary account, the 14-year-old Richard II 'went to Westminster, where he and his lords heard Mass in the abbey. In this church there is a statue of Our Lady, in which the kings of England have much faith. To this on the present occasion King Richard and his nobles paid their devotions and made their offerings'. Richard and his men then proceeded to put the rebellion down, the king silencing Tyler's hordes with the famous words: 'Gentlemen, what are you about? You shall have me for your captain: I am your King, remain peaceable.'

The Wilton Diptych is, you might say, a memorial to the contract which Richard believed the Virgin to have made with him back on that day in 1381: he had prayed to her image and she had rewarded him with victory over those who had threatened his sovereignty. Although the picture was painted in the late 1390s, it depicts Richard II as he might have looked in 1381: the king in the painting is a beardless, fresh-faced adolescent.

The painting might be described as a form of propaganda - a picture which deliberately harks back to virtually the only incident in Richard's reign that could be described (from his point of view) as a political success - but without, perhaps, the cynicism of most propaganda. Richard's faith in his own blessedness seems entirely genuine.

This image of royal piety contains within it an entire world of popular medieval piety that has long sunk from view in this country, submerged by the great tide of the English Protestant Reformation: a world where people believed, fervently, in the power of prayer to incite divine intercession in the affairs of men; a world where not only kings but common people would plead with images of saints or of the Virgin to deliver them from their troubles. The Wilton Diptych seems very likely to have been painted as a devotional image for the private use of Richard II, a portable altarpiece to be set up and prayed before whenever he chose. This lends it a peculiar but affecting, self-referential quality - looking at it, we see a painting of a king praying, before which a real king once kneeled and prayed.

This may partly explain why the figure of Richard in the Wilton Diptych seems so moving. In this frail and porcelain-skinned adolescent we can see an extinct English religious culture, firm in the faith that heavenly intercession was a real possibility - that beings in the other world that is Heaven, the picture's rich and tapestry-like garden, might help them in the wilderness of the real world.

History has given the Wilton Diptych additional poignancy, because we know (as Richard did not, when he commissioned the painting) that heaven would eventually let the boy king down. On 29 September 1399 Henry IV seized the throne. Richard was confined in one of Henry's fortresses in Pontefract, deep in the Lancastrian heartlands. And there he was murdered: starved to death in a castle probably not unlike the one we can now, once again, make out in the middle of the island in a silver sea which occupies two square centimetres of medieval England's greatest painting.