One of the most important of American self-portraits, the famous rondel painted in the early 1780s by John Singleton Copley, the first American master, has been purchased by the National Portrait Gallery of the Smithsonian Institution.

"The painting," said director Marvin Sadik, "is the cornerstone of our collection, and the most important picture acquired by the Gallery since it opened to the public in 1968."

The Copley cost $225,000. The Smithsonian Institution, using "private" trust funds, put up half of that amount. The remainder was provided by the Morris and Gwendolyn Cafritz Foundation of Washington. Though Gallery officials declined to discuss price, a Foundation spokesman acknowledged a contribution of $112,500 towards the purchase price.

Some old portraits are admired because they are well-painted, while others are esteemed for the faces they portray. The Gallery's new Copley, a first-rate picture of a first-rate paintter, is superior on both counts.

Moody and romantic, it is both a fine example of what, 200 years ago, was avant-garde painting, and the finest likeness left us of that ambitious, gifted man.

Copley was born in Boston, then a small Colonial port with a few artists and no galleries, in 1738. By the age of 20 he was painting pictures better than any he had seen.

They were more lifelike, accurately detailed, and more sharply focused. Most American painters of the time were naive provincials, mere sign painters or limners, but Copley had an almost photographic eye.

Sir Joshua Reynolds was so favorably impressed when he first saw work by Copley that he wrote to tell the young American to escape from Boston "before it was too late." Copley took the Englishman's advice and left the Colonies for London in 1774.

There his style changed. His paint handling grew looser, his pictures more ambitious, and he became a leading member of England's avantgarde. The Gallery's new portrait --with its dramatic lighting, its intensity of emotion, and its spontaneous brushwork -- exemplifies the new romantic manner that would become the dominating style of early 19th-century art.

Copley was not only the outstanding American painter of the 18th century.His early London paintings, among them "Watson and the Shark," now at the National Gallery of Art, are, writes scholar Frederick Cummings, "the most significant early expressions of British romantic art." Except, perhaps, for Benjamin West, a much less gifted artist, Copley was the first American to contribute to the mainstream of Western European painting.

The round self-portrait, 18 inches in diameter, was once in the collection of the late Mrs. Gardiner Greene Hammond (the former Esther Fiske) of Montecito, Calif., who died at 87 in 1955 Though Sadik did not name her, he said the picture's most recent owner was a "West Coast private collector in whose possession it had been for many, many years."

Sadik was attempting to borrow the picture for an exhibition when he learned it was for sale. "Like everyone else who has ever seen this painting," he said, "I have always been staggered by its power and beauty. When I learned it was going to be sold, I decided the National Portrait Gallery had to have it."

"We didn't have a nickel, but we managed to raise the funds. To get it was just flabbergasting. There is only one other American self-portrait that I admire as much -- the Thomas Eakins in New York at the National Academy of Design.

"Two years ago," he continued, "we began to design a medal for presentation in the fall at our 10th anniversary celebration. We put the Copley on the medal, though at the time we did not know that we would ever own it. We picked it because it is the quintessential American self-portrait by the first great American painter. It is also true that we chose the Copley for the medal because both of them are round."