In 1963, when his covered wagon left Fort Bridger, Colo.; for the goldfields of Montana, William Andrews Clark was poor, 24 and anxious about Indians. He survived the Shoshone. Then he struck it rich.

By 1925, when he died without his boots on in his $7-million, 130-room mansion on Fifth Avenue, he had become, in sequence, a prospector, a merchant, a robber baron, a copper king, a one-term U.S. cenator, and an art collector, too.

The old and precious things he bought went on view last night in a delightful exhibit at the Corcoran Gallery of Art.

Those who know the Corcoran as this city's most adventurous museum of moder art should see how it looks now.

Suddenly it seems an historical museum. Its columned halls are filled with works by the Dutch masters, with antiquities from Greece and Rome, with luxurious Persian capets, with harpsichord from France. That this display surprises is self surprising. Many of these objects have been here all the time.

Clark, upon his death, willed 800 works of art to his Fifth Avenue neighbor, the Metropolitan Museum of Art. But the Met, deciding not to build a wing to house them, turned his bequest down. New York's loss was our gain. The Corcoran in Washington was the senators' second choice.The new W.A. Clark wing, paid for by his family and filled with his collection, opened to the public here in 1928.

It was designed by Charles A. Platt (who was also, incidentally, the architect for Washingtons' Freer Gallery of Art.) The Clark wing at the Corcoran was built behind the atrium. And those who come to see the Corcoran's exhibits of new and local art frequently neglect to penetrate that far.

The Corcoran, for years, has seemed schizophrenic, half flashy and half stodgy. Jane Livingston and Edward J. Nygren, its curators, have diminished those divisions. Because her exhibits of contemporary work are as sensitive to history as Nygren's shows of older things are to the present, the Corcoran today seems somehow whole again.

Nygren did the Clark show. He is 43, a Yale Ph.D. who joined the Corcoran 18 months ago. His installation is marked throughout by scholarship and wit.

Look, for instance, at the works that he ahs placed in the rotunda. The "Hope Venus" stands in marble modesty at the center of the room, gazing at the just-as-naked "Bacchante" of Corot hanging on the wall. An allied painting by Fortuny Y. Marshall, a 19th Century Spaniard, hangs just around the corner. It is a frothy, saucy picture in which an artist's model shows a group of leering gentlemen her rosy, ample charms.

A catalogue essay by Douglas Lewis of the National Gallery of Art tells us that the "Hope Venus" is a copy. But her pedestal is original. It was designed by Thomas Hope whose brother, Henry Philip, gave his name to the Hope diamond. The Corot that hangs beside her reminds us of the senator's fondness for Barbizon painting. These works, and the Fortuny, Tell us something too, of genteel last.

Sen. Clark, though courageous and sophisticated, was not entirely respectable. He ran thrice for the Senate. He lost his first race. He won his second, but his victory was canceled when a committee of the Senate, troubled by the many bribes he had distributed, declared his election "null and void." So, in 1900, Clark ran once again. This time he got in.

His collection, too, reflects both his personality and the standards of his time. Not all of it is splendid, much of it seems purchased primarily to decorate his huge and ornate house, but here and there one stumbles on an undisputed masterwork.

There is a tryptich by Sienese master Andrea Vanni that David Brown of the National Gallery calls "his finest surviving work" One Isfashaw "Vase Capet" in the Clark collection is among the finest Persian carpets known. There are two Jan Van Goyen landscapes that would grace the Dutch collections of any great museum. Clark bought Degas well, and Blakelock, Gainsborough and Daumier. Unfamiliar treasures are scattered through this show.

Though he bought few American pictures. Clark was from the start an enthusiastic supporter of the Corcoran Blennials. In 1921, he gave the Corcoran $100,000, the interest to be used for Biennial prizes. In 1927, his widow gave the gallery a similar amount. A remarkable set of pictures, all purchased in whole or part with the modest funds left over after Beinnial expenses, also are displayed.

These include works by Hopper, Tack, Albers, Ansel Adams, Walker Evans, Gilliam and Krebs. Nygren points out that just one of these paintings - George Bellows' exhuberant depiction of "Forty-Two Kids" swimming - is alone today worht more than the $200,000 given in cash by the Clarks.

The Clark show begins in the 8th Century B.C. and takes us to the present. It reminds us that the Corcoran has, in its permanent collection, an art historical resource of enormous value. The exhibit, dedicated to the late Dorothy W. Phillips (1906-1977), former curator of collections, closes on July 16.