ART; FROM MANY ASPECTS

IT has been compared to one of Hitler's bunkers, a concrete doughnut, and the turret of a tank. It is the Hirshhorn Museum, a starkly modern presence on the Washington Mall which houses a magnificent collection of contemporary art.

The Hirshhorn Museum and adjoining Sculpture Garden is named for the late multimillionaire philanthropist Joseph Hirshhorn, a Latvian emigre who came to the United States as a child of six. As a boy, Hirshhorn discovered art on an insurance-company calendar full of Bouguereaus and Landseers, pasting the pictures on his bedroom wall and studying them, museum Director Abram Lerner writes in an introduction to the museum.

Later, as a successful businessman, Hirshhorn began buying the originals, the first step in his 40 years of art collecting which spanned a rich variety of styles and periods. The collection - which currently includes 3,500 paintings, 2 ,000 pieces of sculpture, and 2,000 works on paper - is now conservatively valued at $100 million.

It is housed in a circular building of tan reinforced concrete 231 feet in diameter, by architect Gordon Bunshaft of Skidmore, Owings & Merrill. The building, itself designed like a contemporary painting or sculpture, embraces the collection within its intersecting curves and arcs. At the entrance, Henry Moore's bronze ''King and Queen,'' regal but barefoot, sit enthroned as part of the largest public collection of Moore sculpture in the nation.

The collection includes sculptures by Picasso, Louise Nevelson, Alberto Giacometti, Claes Oldenburg, Degas, Rodin, and David Smith, as well as the bright, twirling mobiles of Alexander Calder.

''He never bought what he did not respond to,'' Lerner wrote in his guide to Hirshhorn's collection. Visitors can see the stamp of Hirshhorn's taste in paintings as different as Thomas Eakins's unflinching portrait of his wife; Mark Rothko's dynamite charge of color, ''Blue, Orange, Red''; or Larry Rivers's 33 -foot-long ''assemblage,'' a wall of visualized ideas titled ''The History of the Russian Revolution from Marx to Mayakovsky.''

The paintings, hung mostly on vast expanses of white wall, radiate and shimmer with the colors of contemporary greats like Hans Hofmann, Jackson Pollock, Morris Louis, Milton Avery, Frank Stella, Helen Frankenthaler, Josef Albers, Rene Magritte, Joan Miro, as well as earlier artists like Winslow Homer and John Singer Sargent.

The diversity of the collection, for instance, puts Man Ray's ''Blue Bred,'' a sculptured hunk of blue plastic French bread, across from Childe Hassam's dreamily romantic ''Leda'' with its legendary swan. Final advice to visitors: Take notes only with pencils since pens and markers are under a ban enforced by guards. And when you've finished your tour, on the third floor, plop down on the expanse of black leather couches facing a 180-degree view of Washington in a room full of vibrant Miros and Calders.