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Leonardo da Vinci: Painter at the Court of Milan

National Gallery, London, until 5 February 2012

Leonardo arrived at the court of Ludovico Sforza in Milan in around 1482 as a musician, perhaps bringing a lyre as a gift from the Medicis. Yet the polymath's early unfinished portrait of a musician has been plausibly interpreted as an assertion of the superiority of painting over music, the song on the young man's lips lost forever but his image preserved in oil.

This small but astonishing exhibition brings together nine of the 10 paintings Leonardo produced during his most prolific years in Milan (The Last Supper obviously couldn't be moved), representing well over half his total surviving output. The organisers' greatest coup is to have persuaded the Louvre to lend its earlier version of the Virgin of the Rocks, here displayed face to face with the National Gallery's: the recent cleaning of the latter makes the landscape seem far less ominous, the characters' faces apparently illuminated from within. The curators have even uncovered a sketch where the Virgin is kneeling to fit into the space on a piece of paper tightly packed with other figures, which may have influenced her pose in the finished paintings.

Early in the exhibition we are confronted by two supreme female portraits, Lady with an Ermine and La Belle Ferroni¨¨re. The former represents Cecilia Gallerani, Sforza's 16-year-old mistress; the latter probably depicts his wife. They are accompanied by sketches showing motifs that fed more or less directly into the paintings, but also, almost cruelly, by three portraits from the artist's school: one manages to make a young man look like a raddled transvestite in a Parisian nightclub.

By the end of the period he spent in Milan, Leonardo had forged a distinctive artistic style that helped the Sforzas to claim that they had created a new Athens. He had also founded what amounts to a school, with talented disciples such as Giovanni Antonio Boltraffio producing fabulous sketches of drapery and breast-feeding babies, as well as variants of Leonardo's Madonna Litta.

There may never be another opportunity to see so much of Leonardo's artistic output together in one place. His scientific interests make a fleeting appearance in some of the sketches accompanying the ambitious unfinished portrait of an anguished St Jerome, much of it still in outline, although the taut muscles and the dark cavernous space below the chest of the kneeling saint remain powerfully emotional. The first item on display, however, is a sketch of a human brain with its different chambers, plus layers of scalp. On the back, rather eerily, is a quick impression of what is clearly the same man as a sage. Miraculously adept at capturing the beauty of surfaces and their spiritual significance, Leonardo is never afraid to examine the skull beneath the skin.