The restaurant is booked for weeks to come, the white oak floors are already scuffed and the queue for the bridge to Millennium Park is impossibly long.

Renzo Piano's $300 million Modern Wing at the Art Institute of Chicago opened just two weeks ago, and Chicagoans instantly embraced it as their own. Besides suites of skylit galleries, a restaurant, a shop and a vast atrium, the glamorous new wing frames mesmerizing views of the popular park and the skyscrapers overlooking it. Within the first week, thousands of visitors took advantage of free admission to see whether the expanded building lives up to its new status as the country's second-largest art museum after the Metropolitan in New York City. It does.

The Piano addition is a sophisticated essay in understated elegance -- graceful, poised and self-assured. Built on the southern edge of Millennium Park, it consists of a pair of glass pavilions flanking a three-story atrium that ties into older sections of the museum complex.

Slender ribbons of limestone and aluminum frame a canopied entrance facing the park. A louvered roof known as the "flying carpet" floods the top-floor galleries with baffled sunlight, while walls of glass and translucent screens bring natural light into exhibition halls and classrooms.

Light is too often shut out of museums, banished as a destructive agent that can literally cook the art by leaching color, drying wood and breaking down canvas. The new technologies that welcome light into the Modern Wing completely transform the museum experience, allowing visitors to experience subtle, even playful new harmonies between nature and culture, as when a Giacometti sculpture appears about to stride out of a third-floor gallery into the garden or a Picasso figure is seen lolling on a hummock like those across the street. Views of the lush, green park and sparkling Lake Michigan refresh the eye, reorient the mind and relieve the often claustrophobic and disorienting experience of wandering through acres of art.

The soaring atrium reinforces the relationship between the museum and surrounding city. Arriving from the street or park, visitors enter a three-story cathedral of white walls, pale wood and glass. The first floor holds a gallery for temporary exhibits, a stylish shop and classrooms with persimmon-red floors that will awaken even the most reluctant field-tripper. Long flights of suspended stairs rise to galleries in the East Pavilion and to a balcony coffee bar that links the two pavilions and ties into older sections of the museum. The view from the balcony is spectacular, embracing an urban vista that runs from neoclassical and Art Deco skyscrapers to the froth of metallic waves topping Frank Gehry's amphitheater in the park.

Pride of place

The Chicago museum is best known for its unparalleled collection of late 19th-century French Impressionist paintings, but the Modern Wing makes clear that it has strengths in 20th-century and contemporary art, too. Its justly famous European-modern collection (1900-50) gets the airy top-floor spaces in the East Pavilion, where the silvery light on a bright summer afternoon is an almost palpable presence.

Paintings and sculpture are sensitively arrayed with attention to color, form and period. After the gray-brown Picasso/Giacometti gallery comes a burst of bright biomorphic paintings followed by rooms of surrealist and sexually suggestive images by Balthus, Magritte, Paul Delvaux and Salvador Dali, whose plaster Venus de Milo with strategically placed mink pompoms is a sure hit. There's also a glorious collection of sleek Brancusi sculptures.

Surrealism and early 20th-century figuration exerted an outsized influence on the development of Chicago's own funky "imagist" school of painting. Max Ernst sketchbooks, a Francis Picabia portrait made of burnt matches and hairpins, and a wall of Joseph Cornell's dreamy boxes are followed by rooms of Braque and Beckman, De Chirico and Bonnard. No mere roll call of 20th-century masters, many of these are signature images, including Matisse's great 1909 "Bathers by a River" and that dorm-room favorite, Picasso's "Blue Guitar."

Below on the second floor, the "contemporary" holdings are divided somewhat awkwardly into two periods: 1945-60 and after 1960. While the designation is common in museums, it's a strain to call 60-year-old art "contemporary." The immediate post-World War II era is not well represented, as seen in drive-by paintings by Willem de Kooning, Barnett Newman and Jackson Pollock. And the curators went seriously wrong in hanging a wan Mark Rothko near a dynamite, sponge-festooned Yves Klein whose iconic blue pigment utterly obliterates Rothko's washed-out yellow and orange.

The 1960s-'til-now galleries are much stronger, with dynamic paintings by Lari Pitman, Ellen Gallagher, Marlene Dumas and especially Kerry James Marshall, whose five-image "Vignette Suite" incongruously injects Black Power tension into a Fragonard seduction theme. Charles Ray's 2007 sculpture "Hinoki," which reproduces a dead cypress tree in microscopic detail, is surprisingly haunting. However, the continuous showing of Bruce Nauman's "Clown Torture" video is just that -- torturous. And it is extravagant to devote two capacious galleries to Robert Gober's hectoring wallpaper about lynching and sculptures about childhood neuroses.

On the opposite side of the atrium, the second floor of the West Pavilion holds the architecture and design galleries. They currently offer an amusing sample of contemporary furniture and an impressive show of architectural drawings by such modernist heavyweights as Eliel Saarinen, Le Corbusier, Paul Rudolph, Richard Neutra, Louis Kahn and an amusing 1950 Bruce Goff drawing of a bizarre structure made of nets and suspension bridges that looks like a macrame project run amok. Note also the "Corallo Bench," a fantastic sofa made from scrunched-up orange wire.

Circulation trouble

While the galleries are perfection, the Modern Wing's circulation is peculiar. The new 160-seat restaurant and outdoor sculpture court are on the third level of the West Pavilion, which can be reached only from the atrium lobby. The floor plan hints at stairs and an elevator behind the shop, but the lines were too discouraging to make the trek.

The wing's opening also prompted a vast, and not very happy, rehanging of the rest of the museum. The famous Impressionist paintings are now relegated to a long corridor of smallish rooms spanning the railroad tracks that separate the original Michigan Avenue building from the newer wings.

The chronology falls apart with no clear links to the new section, and too many older galleries have been painted shades of boardinghouse beige. Yucko. Where are the peach and yellow, the summer greens and French blues of yesteryear? Why are the Dutch portraits and luscious Tiepolos and the 17 Monets all backed by dreary dun and tatty taupe that sucks the breath from their souls? The Art Institute's collection is rightly the pride of Chicago, and it all deserves treatment as felicitous as that in the Modern Wing.

Mary Abbe • 612-673-4431