"The Armand Hammer Collection" at the Corcoran Gallery of Art is not the same one shown here a decade ago.

In its present incarnation, Dr. Hammer's art collection is a trove of treasures. But when last seen in Washington, it was laughed at for its losers.

Since that sad show closed, Hammer has acquired a set of master drawings -- by Leonardo, Durer, Michelangelo and Fragonard, Raphael and Rembrandt, Seuarat, Degas and van Gogh -- that includes some of the finest sheets still in private hands.

Among the oils he has bought since tghen are pictures just as grand: the two Moreaus; the Cezanne; his scarlet Sargent portrait; a gemlike Harnett still-life; "Juno," hsi $3.25-million Rembrandt; and "Hospital at Saint-Remy," the $1.2-million van Gogh.

The pickets from Environmental Action -- dressed in trash cans and gorilla suits -- who yesterday cavorted at the front door of the Corcoran may insist that Hammer has not done enough to clean up Love Canal. But there is no doubt has cleaned up his art.

The chance to see in Washington, say, that Ingres portrait of plum-faced Mrs. Badham with Rome in the background behind her sketchy shawl; or that perfect Harnett still-life; or the way that Rembrandt suggests with a few brushstrokes the gold of Juno's sleeve, is enough to warrant a visit to this show. The handsomely installed Hammer exhibition, which was previewed last night and opens Saturday to the public, is a feast for the eyes.

Once, when buying pictures, Hammer shopped for bargains. But since he learned his lesson, he has paid top dollar -- which, being a multimillionaire, he can well afford. Once he trusted his own eye. But in 1970, when he was still smarting from the critics' pans, he began to turn to experts -- particularly John Walker, the man who used to run the National Gallery of Art.

"I had spent 30 years trying to avoid having to tell collectors about their poor judgement, and I wasn't going to begin now," Walker writes of Hammer's request for help. But Hammer insistent -- "he begged me," Walker says -- and Walker came around.

It was he who walked through Hammer's show, at Hammer's invitation, ruthlessly -- and rightly -- combing out the junk.

The quickly formed collection was peculiarly displayed in 1970 -- behind the giant elephant at the Smithsonian Institution's Museum of National History. "As soon as I saw the show," writes Walker, "I realized that about half the pictures were mediocre, a few unauthentic, but a number of acceptable quality with a scattering of masterpieces." The few masterworks he saw then -- Rubens' charming portrait of a "Young Woman With Curly Hair," Fantin-Latour's "Peonies," Cezanne's "Ecorche" -- are still in the collection. But the lesser works are gone.

There was a wall of Corots in the 1970 exhibit. (It was Corot of whom it was once said, "he painted 1,000 pictures, of which there he 3,750 in the United States.") Of the 18 Corots in the 1970 exhibition, 14 have been discarded. The Cezanne landscape in that was a mess by any standards. It was bought in 1880 by an amateur collector who, believing it "unfinished," asked Cezanne to touch it up. The artist tried, but failed, and spoke complainingly thereafter of his picture's "execution." The "Boy Resting" by Cezanne, with which Hammer soon replaced that ruin, is a vastly better work. He has got rid of two Monets, three crummy Renoirs and both Winston Churchills. Cutting from the bottom, he has thrown out half the show.

As soon as the 1970 exhibit closed, he set out on a 28-month buying binge, during which he purchased many of the finest pictures at the Corcoran.

By then, he would not buy an oil unitl Walker had approved.Two other gifted scholars, Konrad Oberhuber and Christopher White, both then employed by the National Gallery, helped him pick his master drawings -- all of which he has pledged to that museum.

In May 1970, he paid a reputed $150,000 for his fine Thomas Eakins "Portrait of Sebastiano Cardinal Martinelli"; in June, he bought his Caillebotte oil and his Manet drawing; in July, he bought a van Gogh, in August, a Degas. That October he purchased two Wateaus, four grand Fragonards, his amazing Ingres, another Degas, another Monet and a Maurice Pendergast. He bought drawings by Correggio and a Giovanni Battista Tiepolo in November, and before the year was out acquired two major oil portraits -- one a first-rate George Washington-with-rainbow done by Gilbert Stuart; the other John Singer Sargent's life-size portrait of "Dr. Pozzi at Home" in his scarlet robe.

In 1971, adventurously, he bought those two Gustave Moreaus, one of Salone dancing, the other of King David brooding. It was of Moreau that P. de Saint-Victor wrote: "If an opium fiend could translate his visions into reality with a goldsmith's skill, it would give some idea of this artist." i

The Harnett and Durer's "Tuft of Cowslips," two gems of his collection, were bought in 1971; the Leonardo drawing in July 1972. In the 2 1/2 years after his Smithsonian show, Hammer spent more than 4 million upgrading his collection.

Hammer, a Corcoran trustee, said yesterday that he was negotiating with the Poles for a loan of major works from Poland's state collections -- among them "The Lady with the Ermine," the Leonardo masterpiece that now hangs in Cracow. Hammer said he hoped that these works would be displayed at the Corcoran in the same skylit galleries, recently refurbished at his expense, where his collection will remain on veiw through Nov. 30.