The latter is also a mysterious exhibition, for its theme is the ancient conundrum of the relationship between sculpture and painting, a puzzle that now seems deeper than it was before. We have always known that Caro is indebted to painting. The sculptures in Trafalgar Square have been chosen to make that debt explicit - and yet, the workings of Caro's imagination are still unfathomable.
Influences and derivations are explained to us via the catalogue, and numerous photographs and placards. Caro's lyricism still bounds away from his sources. The plain fact is that his sculptures do not resemble the paintings on which they are said to be based. At the National Gallery is a sculpture with the title Dejeuner sur l'herbe II. We are referred to the painting by Manet. The exhibition strongly suggests that the sculpture is a transcription of the painting. But does it help the viewer if he or she tries to match one part of the sculpture with one part of the canvas? I think not.
If we do this, imagining that it might be an enlightening or interesting exercise, then the integrity and personal beauty of the sculpture is diminished. It succeeds on its own terms. This piece has such grace and lucid complexity that we should place it among Caro's masterpieces. The real exercise for the eye is to appreciate the work for its own sake and to compare it with, for instance The Triumph of Caesar, a superb but still a lesser work.
Dejeuner sur l'herbe II belongs to 1989, and was shown that year in an exhibition in London which had an introduction by the artist's old friend, Richard Rogers. Without being too specific, Rogers drew parallels between Caro and a number of contemporary architects. It was rather a rueful bit of writing. "The architect is at best only half an artist," Rogers said; mainly because the architect has to serve ordinary needs, in particular that of providing shelter. "When I am struggling to meet the utilitarian demands of a particularly difficult client," Rogers confessed, "I envy Tony his freedom."
Well, Caro has now found himself a difficult client: the public. And he has entered the field of architecture with his first utilitarian project, a bridge over the Thames which he has devised with the architect Norman Foster. This is the subject of a subsidiary exhibition on the lower floor of the Juda gallery.
Architect's models never have the aesthetic interest of sculpture, but these are special because the enterprise is so forward-looking. The Millennium Bridge is the first new crossing of the Thames this century. It's for pedestrians, will span the river between St Paul's and Bankside, and is planned for completion by May 2000. As one would expect, the technology is in the span and the piers of the bridge itself, while the "termini" on the north and south banks will essentially be huge Caro sculptures. Thus Caro's "sculpitecture" of a decade ago will now have a practical outcome. How pleasant to have this bridge, and how stimulating to think that it will afford views of London and the Thames that have never been seen before.
Upstairs at the Juda gallery are large new sculptures in stoneware, steel, wood, and a combination of these materials. Requiem is the most massive of the big pieces. It has a heavy, dolorous presence not quite like anything we have previously encountered in Caro's art. The feeling that it has been built rather than assembled surely comes from Caro's study of architecture. There is however no overt architectural reference, as there was in Caro's "sculpitecture", with its windows, staircases and landings. Requiem is not pierced at any point, so forbids the spectator to consider interior space. Caro hasn't done this before, or not, at any rate, with a large-sized sculpture.
Impressive though Requiem is, many visitors will remember this show for its "book sculptures", small and even tiny sculptures full of playfulness and delight in paradox. They are a further development of the "writing pieces" that Caro has been making since 1978. The difference is that they all begin with stoneware, to which steel or brass is added. I do not know what gave rise to this invention, but the results are delectable. First, Caro made stoneware shapes in the Grasse studio of the ceramist Hans Spinner. They all had the general look of books, usually half opened. Then, these somewhat primordial shapes were brought back to London and given astonishing life by the addition of small pieces of metal. The books remind us that Caro is a miniaturist as well as a monumental sculptor. Probably they need to be so small and humorous. If the books had a larger size they would enter a different and more worrying emotional territory.
This is precisely the area occupied by Caro's variants on Van Gogh's painting The Chair in the National Gallery. These sculptures also combine stoneware and steel. Within Caro's personal artistic history, they are descendants of his "Trojan War" sequence of 1993-94; but these pieces came about when the National Gallery recently invited Caro to make a sculpture related to a picture in the collection. The resulting works are not so dramatic, nor so eloquent, as Caro's reinventions of Greek history and myth. They strike me as mute and unhappy. No doubt Caro was thinking about the terrible circumstances in which Van Gogh painted his picture. But he cannot have been thinking only of Van Gogh, for in these sculptures there is also a reminiscence of Picasso.
We still do not know - nor ever will - what the trigger is that makes Caro invent original three-dimensional art from his way of looking at paintings. It's a large subject, with this inexplicable matter of intuition at its centre. I think that Caro's relationship to painting may be most potent when
it is least recognisable. Other sculptures in the exhibition are related to Rembrandt, Mantegna, Goya and Giotto. They are not well displayed, and the explanatory placards are even more intrusive than in other National Gallery temporary exhibitions. But at least there's no admission charge.
NG, WC2 (0171 839 3321), to 4 May; Annely Juda, W1 (0171 629 7578), to 18 Apr.