Art: Holbein's inner game

The weather was freezing, his lodgings were damp and he was unable to shake off an apparently interminable cold in the head. "I am, and have been, very weary and wearisome," he wrote to his family. "I am the most melancholy, weary and wearisome ambassador in the world." 1533 was a bad year for Jean de Dinteville. But with the help of Hans Holbein he managed to put a brave face on it.

Dinteville is the man on the left in Holbein's recently restored masterpiece The Ambassadors, wearing the salmon-coloured satin shirt and the black silk gown lined with lynx fur. This was probably the outfit he bought to wear to Westminster Abbey as France's representative at the coronation of Ann Boleyn, Henry VIII's new queen; and he resented how much attendance at this inauspicious ceremony cost him. "I shall have to go to great expense for this coronation," he complained in another of his gloomy letters home. Holbein, who was himself something of a diplomat, portrayed Dinteville with a countenance unclouded by any such mundane concerns. Wary and self- possessed, the French ambassador stares out from the canvas as if daring us to guess his thoughts. Those of his companion, Georges de Selve, the Bishop of Lavaur, seem even more cloaked. He is the living image of inscrutability. Impassive and sombre, there is a secretiveness about him which extends even to his posture. He holds his long damask robe about him almost defensively. He leans on a closed book, which seems appropriate since he presents himself to us as one, too.

Holbein's double portrait is a carefully constructed visual puzzle devised to test the ingenuity of those who would decode it. Almost every detail has the tantalising character of a clue, hinting at hidden significance: the closed green curtains that form the backdrop to the scene; the often unnoticed silver crucifix they almost conceal in the top left-hand corner of the picture; the scientific instruments; the globes; the books; the lute with the broken string; the famous blur of the skull, rendered in anamorphic perspective, hovering over the elaborate Cosmati-work tiled floor.

Now that the somewhat artificial furore provoked by its conservation has died down - most of those who initially criticised the restoration, including that professional curmudgeon Brian Sewell, concede it has been a success - the National Gallery has decided to devote an exhibition to the picture and its meanings. This may be construed as a gentle hint, an invitation to stop worrying about the physical condition of the painting and to enjoy it again as a work of art. The restoration matters only because it has made it easier to see The Ambassadors for what it is: a painting which contains, locked up within its still surface, the wracks and tumults of an extraordinary moment in history - an image, not just of two men alone with their thoughts in mysterious circumstances, but of Western Europe in the throes of a great religious and political crisis.

The Ambassadors was commissioned from Holbein by de Dinteville to hang in his family's chateau at Polisy. One of the earliest of many Italianate chateaux erected in France during the reign of Francois I - a courtly building boom which eventually produced that long line of fantastical castles stretching along the Loire valley - Polisy was gutted by fire in 1992. It is therefore impossible to reconstruct exactly where or how Holbein's painting would have hung, but the fact that it was designed for private contemplation is an important clue to its character. The Ambassadors is the pictorial equivalent of a Renaissance nobleman's cabinet, the most precious piece of furniture in the house, a repository of secrets and of special knowledge reserved only to those who understand the trickery of its construction.

The composition of the picture is even literally reminiscent of a cabinet. The two men leaning on their elbows are like human doors, that have swung open on their hinges to reveal a mass of intriguing bric-a-brac stacked on the shelves between them. This has been the matter of their contemplation, and now it has been opened to our gaze. But do we have the means to interpret it?

Thanks largely to the pioneering detective work of Mary Hervey, a dogged and tenacious historian who established the identities of Holbein's sitters almost a century ago - and whose findings are admirably summarised and developed by Susan Foister in the excellent catalogue to the National Gallery exhibition - we have in fact been furnished with keys to many if not all of the picture's secret compartments. Restoration has, furthermore, clarified some of the more recondite aspects of its symbolism (see overleaf).

We know that Jean de Dinteville met Georges de Selve, Bishop of Lavaur, on 23 May 1533, because he noted the meeting in a letter to his brother - also a diplomat - and told him to mention it to no one.

Diplomats were expected to engage in some espionage and were familiar with codes and encrypted secrets. This may explain, in part, the character of the painting Dinteville commissioned. The only other record of this secret rendezvous is the Holbein. Although it seems unlikely that we will ever know precisely what the two men discussed we do know what was likely to have been on their minds.

Dinteville's mission to London of 1533 took place at a time when the political map of Europe was being redrawn. The ambassador would have been conscious that these were difficult years for his country. France had recently lost all of its Northern Italian territories to the Holy Roman Emperor, Charles V, whose troops had sacked Rome in 1527. Meanwhile, the seemingly uncontrollable popularity of new Lutheran ideas threatened to unbalance the political status quo yet further. France was simultaneously threatened from within by Protestant dissenters and, from without, by the quickfire spread of Protestant ideas to powerful states such as England. The impending division of Europe, as much as the English weather, must have contributed to Dinteville's almost Eeyore-like melancholy.

Dinteville's visitor on 23 May 1533, Georges de Selve, was greatly exercised by the religious divisions of the time. He had spent much of his career on religious-cum-diplomatic missions trying vainly to stem the tide of Lutheran reform. At the Diet of Speyer of 1529, one of many meetings convened in an attempt to reconcile Protestant and Catholic in the German territories of Charles V, he delivered his Remonstrances aux Alemans, a plea to all German Protestants to consider reunification with the Catholic church. He may well have been in London on similar business, meeting with clerics sympathetic to Protestantism such as Thomas Cranmer, and seeking to dampen down the first smouldering fires of the English Reformation.

Dinteville, too, was in England on a partly political, partly religious mission. He had probably been sent to London to impress upon Henry VIII and his advisers that the French would take a very dim view of plans to establish a separate Church of England. But as it turned out, Dinteville arrived too late to influence a monarch who would almost certainly have refused to listen to him anyway. As soon as he got to London, it was plain that Henry had already taken the decision to divorce Catherine of Aragon.

Since Catherine was Charles V's aunt, the Pope, who had been Charles's political puppet since the Sack of Rome, could not conceivably sanction such an act. The only way forward for Henry was to declare himself the head of an independent Church of England and grant himself his own divorce. This is what he did, and that in turn opened the way to the conversion of England to the Protestant faith.

Dinteville's disapproval of the events unfolding before him is almost audible in his letters back to France, especially in his scandalised comments about Ann Boleyn's appearance at her coronation: "This Queen is clearly pregnant." She was indeed, and in September Dinteville, as Francois I's representative, attended the baptism of the child: the future Elizabeth I, destined to become the monarch of an irrevocably Protestant nation.

In other words, the two ambassadors found that they could do no more than witness events that they themselves were powerless to influence. Alone in their room, they are reminiscent of Rosencrantz and Guildenstern - men condemned to bit-parts who can only watch from the wings as extraordinary events take their course centre stage. Holbein's intricate double portrait is both an allegory of their impotence and a consolation for it.

The Ambassadors is history miraculously preserved, a piece of the past made so immediate that it still lives in the present.

`Making and Meaning: Holbein's Ambassadors', National Gallery, London WC2 (0171-747 2512), 5 November-1 February

The objects heaped on to the shelves on which both men rest their elbows (seen in detail, above) symbolise the disarray into which they felt their world had fallen. It used to be thought that the various astronomical instruments on the top shelf, which include a celestial globe as well as a number of different mechanisms for telling the time by the motions of the sun, might indicate some important date relating to their respective diplomatic missions. But cleaning of the picture has established that each one of the instruments to the right of the celestial globe - a cylindrical shepherd's dial, two quadrants, a polyhedral sundial and a torquetum - are all curiously misaligned for use in a northerly latitude. This is unlikely to have been an oversight on the artist's part, since one of his closest friends in London was the astronomer Nikolaus Kratzer (the subject of a small but brilliant portrait in the Louvre) and Holbein can be counted upon to have sought his advice, if not the loan of actual instruments, in the preparation of the painting. He was, after all - it is implicit in his style - nothing if not a meticulous artist. The misaligned instruments are surely emblems of chaos, of the heavens out of joint. The fact that they were intended to be read symbolically is suggested by the generally encrypted nature of the whole painting and confirmed by the lute with a broken string on the shelf below - which was, by 1533, an entirely conventional symbol of celestial discord, and could be found in many emblem books sold to artists with an allegorical turn of mind.

If the theme of the upper shelf is discord in the heavens, most of the objects on the lower shelf refer to anarchy down below, here in the mutable sublunary sphere of human affairs. The celestial globe has been replaced by a terrestrial globe on which may be made out the words "Baris" and "Pritannia" - spelling had not yet been standardised in the 16th century and these are thought to be Holbein's phonetic renderings of his own Germanic pronunciation of "Paris" and "Brittany" respectively. The open book immediately below the globe (and therefore, quite literally, its subtext) is an apparently innocent work of practical mathematics, Peter Apian's New and Reliable Instruction Book of Calculation for Merchants. But it is open at a most un-innocent page, which begins with the word Dividirt: "Let division be made". To those with sharp eyes and the ability to think laterally - or to those let in on the secret by the Dinteville clan, as they contemplated this picture in the privacy of Polisy - this was clearly a reference to the great religious schism that was tearing Europe in half in the 1530s. Division was indeed being made.

The book next to this one is a hymnal open at the "Veni Sanctus Spiritus", a hymn to the Holy Spirit, traditionally invoked as a force for church unification. The dream of reconciliation expressed in this small detail of Holbein's picture would never materialise. The objects speak, therefore, of the wreckage of the ambassadors' hopes and aspirations. But they are not downcast because, as the picture shows, there is another and yet larger scheme of things by which they are content to live their lives and be judged.

At the very bottom of Holbein's picture, we find the most paradoxically blatant statement of its cryptic nature. The unclear object obliquely jutting up from the floor still seemed a complete mystery as recently as 1867 when the then director of the National Gallery, Ralph Wornum, described it as an inchoate shape like "the bones of some fish". Thanks to the researches of Susan Foister and to modern computer technology we know exactly the point at which to place one's eyes in relation to The Ambassadors in order to read this detail as the initiate into the picture's secrets was intended to: at a right angle 120mm away from the wall surface, 1,040mm from the bottom of the picture and 790mm to the right of it. The grinning skull thus revealed represents the most brutal of all facts of life: the inevitability of death.

At the very top of the picture, we find the last piece in the jigsaw puzzle, the detail that fixes all the other details in place and gives them their final meaning. The crucifix in the top left- hand corner is the counter to the skull. The cross stands for the Resurrection, for God's gift of eternal life which awaits all those of true faith. It is from the small truth of a broken world, in the here and now, to this larger truth, under the gaze of God, that the ambassadors turn to stare us straight in the eye. They may have failed in all their schemes and projects, but piously sure of their own redemption, they will retain their admirable sang-froid for all eternity.