What gallery-goers will see today is Holbein's palette of vivid colours returned to glowing life from behind obfuscating clouds of yellowy brown varnish. This was applied to preserve the painting in 1891, the year the National Gallery bought it, for pounds 55,000, from the Earl of Radnor.
Over the decades, the varnish aged, robbing Holbein's colours of their intensity and hiding entire details, including a lute-case under the table and, more significantly, the curiously elongated skull, a memento mori, that rises in spectral fashion between the figures' feet.
Holbein's colours will surprise those used to seeing them through a gauze of Victorian varnish. The celestial globe that stands on the table, for example, and which we took to be a grungy green colour, is now azure blue. The lynx fur edging of Dinteville's robe is dazzling white rather than the tobacco colour we mistook it for.
These revelations have not, however, been entirely popular. Michael Daley, representing the pressure group ArtWatch International, claims that The Ambassadors has been subjected to an "ordeal by swab, solvent and scalpel".
Neil McGregor, director of the National Gallery, denies the claim. "Our duty," he says, "is to present to the public as much as we can of what the artist intended us to see. The first priority is to preserve pictures for the future, the second is to make the experience of looking at them as enjoyable as possible for the present."
This seems fair enough, although it is clear, from as far back as 1891, when The Ambassadors was coated in varnish, that preservation is a questionable business. What makes Mr Daley and art-watchers like him so angry is the belief that we should not tamper with history and the process of ageing. The Ambassadors is 463 years old and what history has done to it is something we ought to recognise and accept. The patina that a painting, or indeed any work of art, acquires over the years is surely a fascinating and, in many ways, a lovely thing.
A fondness for what John Piper memorably called "pleasing decay" is rooted deeply in the nostalgic British psyche. There are as many people who rail against the restoration of a Holbein ("over-restoration", say the accusers) as there are those who say great buildings should not be cleaned. Surely the layers of soot and urban grime that give, say, Nicholas Hawksmoor's Christ Church, Spitalfields, its romantic chiaroscuro appearance should never be scrubbed or sandblasted away?
Classic car buffs can feel much the same way; they will go Ferrari-red in the face if they spot, say, a great Thirties coupe-de-ville stripped of its patina of road-induced age, over-groomed, over-polished and coated in modern varnish.
The same thinking applies to preserved steam locomotives, vintage aircraft, furniture and fabrics. Each of us, it seems, has an ideal picture in our mind's eye of how a particular Persian rug, Ming vase, Great Western locomotive, "Blower" Bentley, Baroque church or renaissance painting should look. Very often these ideal images are formed in childhood, or when we first encounter and fall in love with a favourite painting, sculpture, yacht or car. From then on, we want to preserve that particular image, and if this means a Holbein first spied dark and brooding, then that is how we will always want that painting to be.
Those working in the uncertain field of conservation will always debate the respective vices and virtues of alternative methods of restoration; and each new generation will be delighted or appalled, depending on what it has been brought up to see, by a grimy London parish church on the one hand, or by a sparkling Holbein on the other.
The Ambassadors is a painting that celebrates the intellectual and artistic riches available to renaissance man. It is also a reminder that, however hard we may try to hide it, death is never far away. Both life (colour, texture, sheer vibrancy) and death (that abstracted and disturbing skull) have been restored to their full significance in Holbein's great painting.
Perhaps, though, these are the very things those who like their art crusty do not want to see or to face up to. In its latest guise, The Ambassadors no longer peers out of its frame to a small audience of connoisseurs as if through a glass darkly, but confronts us all as directly as it must have done in 1533.
All we can say with certainty is that the debate between "pleasing decay" and immaculate restoration is a hardy perennial, for there is a lover of the romantic ruin and a begrudging fan of the groomed and polished in each of us.
National Gallery, London WC2 (0171-839 3321) PICK OF THE WEEK Contemporary Print Show Part II
For the second half of this now annual printselling extravaganza, another 10 galleries, including Marlborough Graphics, Curwen Press and the little- known but highly rated Greenwich Printmakers Association, wheel out their wares. Prices go from as little as pounds 50 to pounds 5,000, but even if you don't want to buy, this modest art fair is an interesting free exhibition of some of the country's best printmakers.
Concourse Gallery, Barbican, London EC2. To 8 May (0171-638 4141)
You may think it's all over, but David Trainer knows better. For the past two seasons, Trainer, himself a keen footie fan, has - with their permission - been photographing his fellow enthusiasts across the capital. Now their mug shots have been assembled into an uncompromising exhibition probing our national obsession. Highly emotive, deliberately posed photographs that give the term "swagger portrait" a whole new meaning.
Museum of London, London EC2. Today to 7 July (0171-600 3699)