The tartan Messiah

You know the man on the shortbread tin. It's Bonnie Prince Charlie, who 250 years ago today was routed at Culloden. That much at least is fact. But if it's truth you're after, despite the exhibition's title, you won't find it in the images of the Prince currently on view at the National Army Museum. From biscuit tin to contemporary engraving, they tell a tale of iconographic invention and ideological conflict.

Our image of the prince has been coloured by time and fancy. The Victorians loved him. In JB Macdonald's masterpiece Lochaber No More, he bids farewell to Scotland with a swash and buckle worthy of Mel Gibson. Even in his own lifetime, though, poor Charlie never stood a chance. The portraits here, on paintings, miniatures, prints and engraved glasses, are more than just romantic depictions of a folk hero. They are complex totems and talismans. At their most mystical - if touched by the King or his son, or, in the case of miniatures, if containing a lock of James III's or Charles's hair - they reflected the belief that the Stuart monarchs possessed the power to heal. At the very least they were devotional objects, accorded the status of holy relics. They define the essence of Jacobitism not as the stuff of Scott and Stevenson, but as one side of the essential intellectual conflict of the 18th century, between the "superstition" of the ancient regime and the "reason" of nascent industrialism.

For his followers, Charles Edward Louis John Casimir Silvester Xavier Maria Stuart was not "Bonnie Prince Charlie". He was a new Messiah who proclaimed his father's kingdom. An anonymous print shows him standing before a shield on which the only discernible word is "Christ". In a unique complicating twist, the Messiah is in Highland dress. Charles was well aware of the emotive power of tartan as a national symbol - that it would endear him to his men. More than this, in such imagery lies a deeper significance. Just as Christ was the immaculate spirit of the Godhead clothed in the venal flesh of man, so now Charles, a prince accustomed to silk and lace, was clothed in the rough tartan of that wildest of men, the Scottish Highlander. To the inhabitants of London and Edinburgh, highlanders were no more civilised than the native Americans of the Colonies. For Charles to adopt their dress was a mutual act of faith echoing God's gift of his son: images of him wearing the plaid its testimony.

The extended metaphor of Charles as Messiah was carried through to the ritual use of these images and objects. Large portraits were hung like altar pieces; miniatures worn like crucifixes. Importantly, all were treasonable images, whose possession would result in death or imprisonment. This quasi- religion was further mystified by secrecy. Most obviously ritualistic were the glasses engraved with the Prince's portrait and such slogans as "Redeat" (may he return), "Fiat" (may it come to pass) and "Amen". Drinking to "the King over the Water", Jacobites mimicked the sacrament and whispered: "To King James Stewart. Soil not the holy cup."

Nowhere is the mystery of Jacobite iconography better revealed than in the unique anamorphic portrait on view here. A piece of painted board, when seen reflected in a polished metal cylinder, becomes transformed into a portrait of the Prince in Highland dress. Imagine this fugitive image shimmering in the candlelight amid a chorus of Latin oaths and you perceive the arcane mysticism which really disturbed the Hanoverians. Try as he might, the "wee German lairdie" George II could never match the charisma of Stuart iconography. His polemicists might dream up a similar anamorphic portrait of his own son, Charles's adversary, the Duke of Cumberland. But all they got was an absurdly slender image of a grotesquely fat young man. Sadly, the Jacobites' victory in this war of pictures did them no good against the grapeshot at Culloden.

These objects are neither historical artefacts nor mere likenesses. They are the trappings of a proscribed religion; evidence of a battle between the two truths of "enlightenment" and that of "faith". The real truth, though, is evident in the rheumy eyes of Hugh Douglas Hamilton's portrait of Charles painted in Rome, four years before his death. These are the eyes not of a Messiah, but of a sad old man - a victim of the age of reason.

n `Bonny Prince Charlie, Fact and Fiction' is at the National Army Museum, London SW3 (0171-730 0717). To 28 April