Come up and see these etchings

In New York, few tourists make it to the `Morgan'. In London, a show of the library's drawings reveals what we've been missing

The Royal Academy's new show of drawings, "From Mantegna to Picasso", is the most beautiful exhibition I've seen for months. I recommend it as a delight to eye and mind. All the works on display, by around 60 artists, come from the Pierpont Morgan Library in New York. This makes the exhibition even more of a special treat, for the Morgan is a somewhat retiring museum. There it is on Madison Avenue, but (unless they are scholars) tourists in Manhattan rarely put it on their list of places that must be visited.

Pierpont Morgan (1837-1913) was a railroad and steel billionaire who in his latter days spent his fortune on art and learning - maybe especially learning. It could be said that the heart of his museum is in its collection of books and literary manuscripts. The RA's exhibition, with its emphasis on the purely visual, is really the work of another benefactor, Eugene Thaw. He has been buying top-class drawings for 30 years, and his collection is a promised gift to the Morgan. Thaw believes that the museum needs many more 19th-century works, and this view has influenced his gifts. But he's also up-to-date, being one of the two trustees of the Pollock- Krasner Foundation, which gives some $2m a year to needy artists under the terms of the will of Lee Krasner, Jackson Pollock's widow.

The most recent works on show are not in fact by Picasso, as the title implies, but by Pollock (including one of his sketchbooks from the early Fifties). Personally, I found the most stirring works at the modern end of the exhibition, from the Post-Impressionist period onwards; but people who relish Old Master drawings or English watercolours will find plenty to interest them. Thaw confesses that he has no enormous liking for Italian Baroque drawings, so this period is under-represented (though there are two Claudes). There's nothing from the High Renaissance, but otherwise Thaw has made inspired purchases from most artistic periods from the 15th century onwards.

The earliest works are an anonymous study of draperies from the Rhenish School and a drawing of Three Standing Apostles by Mantegna, whose low viewpoint suggests that this is a study for some grand, but never-realised, mural decoration. Then there's a splendid portrait by Lucas Cranach the Elder, once attributed to Holbein. Thaw says that he is not much interested in attributions, nor in the kind of drawings that are simply useful as art-historical evidence. So the Man- tegna, though it must be explained in art-historical terms, is emotionally powerful, while the Cranach is the first of a number of portraits in the exhibitions. Indeed, the abundance of portraits may offer a clue to Thaw's collecting instincts. He seems attracted to direct displays of personality, whether in portraiture or in the vigour with which an artist insists upon his own vision.

Notable portraits include sheets by Hans Sebald Beham (a follower of Durer), van Dyck, Watteau, Fragonard, Boilly, Runge, Ingres, Gericault, Corot, Chasseriau, Degas, Fantin-Latour, Cezanne, Gauguin, Toulouse- Lautrec, Matisse and Picasso. The 1943 Pollock is a deep, symbolic self- portrait. And these pictures of personalities, occurring at almost regular intervals throughout the exhibition, insist on one message: that drawing is not simply descriptive, nor just a means for planning a painting of a building, but rather a matter of intense human communication. Are drawings of people more searching than their painted counterparts? One is often tempted to think so. Anyway, it looks to me as though Thaw likes drawings by people who have a great and vivid curiosity about other people.

This might explain the relative absence of landscape subjects (and, I suspect, Thaw's less-than- complete love of sculpture). Views of the natural world are provided mainly by Englishmen: Cozens, Constable and Turner. There's a dramatic Moonlit Landscape by Caspar David Friedrich, but I do not count the German's art as a frank response to nature. It is nature treated as ghostly theatre, with the interest primarily in the suggestion of the supernatural. Otherworldly subjects turn up quite often. Rembrandt turns Biblical illustration into gripping mysticism. His Finding of Moses transcends its subject. This wonderful drawing, extravagant in technique, is about light and birth.

Some 18th-century artists liked to observe the real world as though it were governed by irreligious gods. Goya was one of them. His drawings of people he knew or had seen have terror in them. Watteau's irreligious gods were aesthetes. His Seated Young Woman was obviously drawn from life, yet she belongs to an ethereal realm. You can find a painting made from this drawing in the Wallace Collection, with the social additions of a maidservant and a lap-dog. The drawing is more heavenly, though there are brusque passages. Cut it off at the level of the girl's chemise and see how firm her legs are. A bit of a peasant in this goddess.

Watteau's use of chalk allies him to Degas, an Impressionist who could get voluptuous effects from pastel. Renoir's View of a Park is a little troubling. The medium makes it brilliant - bits of it shine and so does the whole; still, it is a trifle vulgar. Granted, Renoir's vulgarity is unique, to be appreciated for its own sake. Still, watercolour is used with perfect reticence, and also majesty, by Cezanne. His three sheets make Renoir look flashy. Gauguin's little watercolour Queen of Beauty is a hot tribute to a new love.

While on the subject of watercolour, note that Van Gogh couldn't use it: his palette was too vehement. So he went back to the brown washes (and the reed pen) of his beloved Rembrandt. Van Gogh's drawing is conspicuously better than a very late work in pencil. This one has the smell of death in it, perhaps because it goes back to his earliest Dutch subjects.

The last portrait subject in this exhibition actually to die was Picasso's mistress Marie-Therese Walter, who committed suicide in 1977. This portrait belongs to 1935, just after the birth of their daughter, Maia. The artist unflinchingly - also helplessly - records the decline of their relationship. To do so, he used traditional, even dated procedures, drawing realistically with pen and ink in wash. This is a disturbing drawing. Thaw managed to get it, but it originally came from the Picasso estate and that is its proper home. But one musn't carp. Thaw and the Morgan Library have made a superb contribution to our general knowledge of art.

! `From Mantegna to Picasso': Royal Academy, W1 (0171 494 5615), to 23 Jan 1997.