The dictionary defines the Enlightenment as a European philosophical movement of the late 18th century, emphasising the superiority of reason as a guide to all knowledge. So, we might expect the Orchestra of the Age of Enlightenment to play the music of late Baroque and classical masters. Yet, today and tomorrow in the Queen Elizabeth Hall, Sir Roger Norrington (right) searches elsewhere for enlightenment when, with the period instrument ensemble, he devotes the latest in his line of celebrated "experience" weekends to the subject of Tchaikovsky. Beethoven, Berlioz, Brahms and Bruckner have all undergone the Norrington treatment in the past, but Tchaikovsky - is he perhaps one bridge too far?

"No, not at all," says Norrington. "The whole point of the exercise works wonderfully well with him. To begin with, it's all too easy to take Tchaikovsky for granted. He's very popular, so, naturally, a lot of so-called 'serious' musicologists tend to pooh-pooh him. Actually, to my mind, Tchaikovsky is one of the greatest of all composers; and, like all great composers, he went forward by looking back, as well as around him. So, although he was an arch Romantic, it doesn't mean we have to play him with these great swathes of sound. The Tchaikovsky Experience should give more than an inkling of what he might have sounded like in his day, when the orchestras of the time were capable of these extraordinarily translucent textures and sonorities."

"One also has to remember that there was really only one generation of Russian composers before Tchaikovsky came along. In order to do what he did, he had to cast his eye on places like France and Germany, which is also where a lot of the players in Russian orchestras of the time came from. And with the players came their instruments, so one might get a mix of, say, French flutes and German valve horns."

But doesn't the potential recreation of that sound today give Norrington a nightmare of anguished choices in terms of the exact dates and origins of the instruments being used? "Well, obviously, the possibilities are almost endless," he says. "But, to begin with, I leave much of the nitty-gritty of it all up to my players who obviously know more about their respective tools than I do. But it's precisely in this area where the period instrument debate gets out of hand. Of course, I'm not saying that we will sound exactly like the St Petersburg orchestra sounded 113 years ago, or what have you. What the Experience tries to do is recreate something nearer to what you might have heard, rather than what we're used to hearing now. Of course, it's not simply a case of assembling the right instruments and away we go. How one plays them is also of major importance - no vibrato in the strings and such like."

For Norrington, as well as for the many ardent devotees of his approach, "...what happens is that we don't make the music sound old, but new. It's all about unleashing these works on the world... again." And the dynamic and forthright Norrington goes straight to the heart of the matter by homing in on two warhorses in the shape of the Pathetique Symphony and the 1st Piano concerto, which will feature Nikolai Demidenko playing the work on a period piano for the very first time. Fascinating and truly enlightening, Norrington's clear reasoning is a perfect guide to our knowledge.

The Orchestra of the Age of Enlightenment perform at the Queen Elizabeth Hall, SE1 (0171-960 4242) today, tomorrow