The Hirshhorn Museum and Sculpture Garden has had few champions as devoted as Marion K. Ring. She loved that round museum. "It is singularly appropriate," writes its director, James T. Demetrion, "that the first private collection to be shown at this institution -- aside from the collection of its founder -- is that of Marion and Gustave Ring."

She taught there from the start. And she learned in turn. In the year before the Hirshhorn opened to the public, she took a six-month training course to prepare her for her work as an unpaid Hirshhorn docent. Though the Rings had been collecting since 1953, most of what they owned -- works by Degas and Vuillard, Monet and Utrillo -- was 19th-century and French. It was only after her exposure to the new museum that they began collecting Hans Hofmann, Adolph Gottlieb, Mark Rothko, Frank Stella and other artists given to wholly abstract art.

In 1978, when the Rings celebrated their 50th wedding anniversary, their friends marked the occasion by giving the museum five still lifes in their honor. With the opening today of "Selections From the Collection of Marion and Gustave Ring," their affection for the Hirshhorn is to some degree returned. The Rings died in 1983, within weeks of one another. The present exhibition, perhaps more touching than important, is a memorial show.

Gus Ring grew up in Washington. The Ring Construction Co. (which he founded here the year he got married) built the Ring Building, the Westchester Apartments, Colonial Village in Arlington, and many other projects both here and in the South. By 1951, Ring had started buying racehorses. That year he also built his wife a house at R Street and Massachusetts Avenue NW, and began collecting art to decorate its rooms. Somewhat like Paul Mellon, Ring came to art through horses; the first object he acquired was an equestrian bronze statuette, "Horse Clearing an Obstacle," an 1885 Degas.

The pictures the Rings went on to buy still suggest the home more than the museum. Most of them are colorful and relatively small. Some of them are splendid. Vuillard's "Portrait of the Artist's Grandmother" (1894); the two still lifes by Georges Braque, and two more by Giorgio Morandi; the early Richard Diebenkorn, "Girl in the Sun" (1957); and the fine Degas pastel, "Bust of a Woman in a Purple Dress" (circa 1885), are among the most impressive. But other objects shown -- the 1966 Miro', the 1936 "Pink Nude" by Matisse, and "Seated Woman" (1966), a small oil by de Kooning -- are, despite their artists' famous names, works of small account.

The "reminiscence" in the catalogue, written by the Rings' family, says that "they acquired art for their personal enjoyment, with little or no regard for its future investment value." Be that as it may, the Rings, or so we gather here, almost never bought "unknowns." Instead they tended to collect what one might call "signature works," objects that we recognize at once were created by big names -- by Henry Moore, Picasso, Marc Chagall, Fernand Le'ger.

Their tastes were catholic. They bought French Impressionists, German Expressionists, the painters of the New York School -- and even a striking, eye-fooling still life, "Five Envelopes With Landscape," painted by Paul Sarkisian in 1976.

Among the nicest works displayed is "G & W" (1944), a small and sweetly colored abstraction by Stuart Davis. It was Marion Ring's favorite. The catalogue tells us "she hung it in her bedroom in Washington, and lovingly carried it to Florida each winter." That sort of affection is felt throughout the show. It closes Jan. 12. John Van Alstine

The Osuna Gallery, 406 Seventh St. NW, is showing recent stone-and-steel pieces by artist John Van Alstine, a former Washingtonian who now lives in New Jersey. He is in perfect command of his materials. He handles rusting steel as if it were malleable as clay. His sculptures never strain. His slabs of corrugated granite are balanced so precisely that their heaviness evaporates until they seem to float.

Van Alstine, it is clear, knows exactly what he's doing. That is the nagging flaw in this seductively good-looking show. Its smooth and doubt-free elegance diminishes its power. Despite its rough-hewn stones, despite its rusty ruggedness, it feels a little pat.

Van Alstine is staggeringly skillful. He is, in fact, so skillful that his skill might work against him. When he began constructing the sculptures in this series, he entered into battle. But now that battle has been won. One leaves this show hoping that a cycle's been completed, that the artist will now search for another challenge equal to his gifts, that he will not long remain here but instead move on. His exhibit closes Nov. 3.