The Madonna with a beautiful secret

This is a story of how a painting once celebrated, and valued above the lives of thousands of men, went mysteriously invisible, even though it was always on display. The painting is called the Wellington Madonna, and it will be seen again next month when the Duke of Wellington's London home, Apsley House, reopens to the public after three and a half years of refurbishment.

You will remember, of course, that Wellington defeated Joseph Buonaparte, then King of Spain, in 1813 at the Battle of Vitoria. Five thousand Frenchmen lost their lives in the action, which ended so precipitately that Joseph was forced to abandon his coach and commandeer one of his troopers' horses. The coach was found to contain a vast stash of canvases cut from their frames, plus panels and other works of art looted from the Spanish royal collections. These Wellington sent home to London. At first he did not realise what a wealth of art he now possessed - 200 paintings, many of them masterpieces. They received expert attention in London, and in 1814 Lord Maryborough, Wellington's brother, wrote saying that in the opinion of Benjamin West, then President of the Royal Academy, and another painter, the Correggio was certainly worth at least 6,000 guineas, and that it and the Giulio Romano "should be framed in diamonds, and that it was worth fighting the battle for them".

As the importance of his acquisitions began to sink in, Wellington wrote to the Spanish authorities to ask if they would like the paintings returned, but the Spanish minister in London at the time conveyed a message that "His Majesty, touched by your delicacy, does not wish to deprive you of that which has come into your possession by means as just as they are honourable." So the paintings were loot no longer. When the Duchess of Wellington produced her sumptuous catalogue of the collection at the beginning of this century, she prefaced it with the relevant correspondence, because by then the Spanish were beginning to regret having lost so many masterpieces.

Wellington loved his paintings, and added to his collection. He loved the Correggio above all, and he kept personal control of a key that opened the glass front of its frame. He used to clean the glass himself with a silk handkerchief, and he treated his Correggio as an icon or personal fetish.

The Giulio Romano, the Wellington Madonna, was a version of one of Raphael's most famous paintings, the Madonna of the Chair. This painting, Raphael's equivalent of Beethoven's Fifth, is a round composition depicting the Madonna and child and St John. It hangs in the Pitti, and was endlessly reproduced.

The Wellington Madonna is a rectangular composition which omits St John. Giulio was Raphael's star assistant - the master loved him like a son, taught him and set him to work on the most important of commissions. The relation between Raphael and his assistants was like this: Raphael as it were owned the "intellectual property", all the ideas and designs were to his credit. But if you said to Raphael, "I liked such-and-such a painting," he might reply, "Oh, I'm pleased. That was executed by my assistant," - meaning Giulio. It was a Raphael, but it was one painted under his authority.

After Raphael's death, Giulio and another, less good painter called Gianfrancesco Penni inherited, legally and artistically, all Raphael's ongoing projects, most of which, Vasari tells us, they were able to complete with much honour. And during Raphael's life, Giulio was able, being a member of the inner circle, to play with Raphael's ideas for his own purposes. He produced six madonnas in this way, making his own variations on the themes which had been entrusted to him by his master.

So if the Wellington Madonna is by Giulio, it is very interesting. But if it is simply an old copy of Raphael's most famous painting, it is incredibly boring from an artistic point of view. Might be a nice thing to own, like one of those plaster reproductions of Michelangelo's slaves. But, essentially, it would be commonplace.

And this was the catastrophe which befell this picture. It began to be called "a good copy of the Madonna della Sedia", and once it was only a good copy, it was finished. People forgot it had ever been by Giulio. Nobody exactly denounced the painting. Its credibility simply evaporated over the years.

But about a decade ago, perceptions must have altered. People looked at the painting and wondered, and among those who wondered was Paul Joannides, author of a catalogue of Raphael's drawings, who had spent a long time pondering the relation between Raphael's work and that of his assistants.

One of the things Dr Joannides perceived in the Wellington Madonna was a beautiful and original idea to do with the lighting of the composition, which comes from two sources. There is a lamp in the background, but the figures are lit from the front. If you were in a darkened room, and you lit a lamp and held it in your right hand and approached the Wellington Madonna, you would have a thrill of perception as you realised the painting's implication that the scene is being lit by the light you are holding. This is a way Wellington himself might have seen the picture (nipping downstairs in his nightgown, just to check) but few have since.

In 1985, Dr Joannides published his ideas, but they did not prosper. However, the V&A, which administers Apsley House, has recently subjected the painting to technical examination and restoration, the result of which has left experts in no doubt. An article by Peter Young and Dr Joannides will be published in the Burlington Magazine in November, and it makes a fascinating read.

What has happened parallels what recently happened in the caseof the Duke of Northumberland's Raphael, which had once, long ago, been considered a Raphael, and then mysteriously turned into a copy, before turning back into a Raphael again. In this case Dr Joannides's instincts of 10 years ago have been illuminated by infrared reflectography, which has shown how Giulio gradually arrived at his beautiful notion: that the original owner of the Wellington Madonna, when he or she approached it, lamp in hand, would take on the role of Joseph, entering and illuminating the scene.

I went round the spanking new Apsley House last week. It has been very well restored, but the Wellington Madonna was yet to be returned to its place beside the Correggio. Whether or not West was right that they are the stars of the collection (there is, after all, that Velasquez), or that they were worth fighting the battle for, it is good to think that his connoisseurship has not gone for nothing. Taste swings around. Paintings come out of their eclipse. West knew things that we are only just coming to know.