Dumbarton Oaks is not a monastery, whatever some people may say, but it has brought me as close to the experience of monasticism as I am likely ever to be. Giles Constable, director, Dumbarton Oaks

DUMBARTON OAKS of Georgetown, that hybrid institution, seems protected by a vow of silence, it is so little known.

It is true that a few people, almost every afternoon, discover the green beauties of its lawns and ancient trees. An even smaller number of intrepid viewers -- after tugging on the oaken doors on 32nd St. -- find beyond that barrier the two sweet small museums of Byzantine and Pre-Columbian art where "D.O.'s" curious treasures of silver, gold and jadeite are permanently displayed. But for most Washingtonians, "America's most civilized square mile" remains a place unknown.

Though it belongs to Harvard and is full of scholars, it has not offered courses or granted degrees. It is an absolutely unique research institution, triangulated oddly among three fields: landscape gardening, Pre-Columbian art and -- particulary -- Byzantine history. For many years Dumbarton Oaks seemed determined to conduct its arcane work in isolation. But now, for the first time -- cautiously but steadily, and too speedily for some -- Dumbarton Oaks has started reaching out to Washington.

Its two small museums used to close in summer; this year, for the first time, they are open every day. The gardens, too, are open, and a telephone recording (338-8278) tells visitors what's blooming there and where to park. Garden tours are being planned, as is a project to re-label the collection. Exhibit spaces may expand, hours may be lengthened, and a class for local students will be offered there this fall.

"I am not naive," says Giles Constable, the Harvard-trained historian who two years ago became the first "outsider" to direct Dumbarton Oaks. "I know that there are poeple who loathe this institution, who regard it as irrelevant, stuffy and elitist. Everything we are reeks of what some people hate."

"Elitism" these days is much in disfavor. But the main work that is done there -- the detailed study of the Byzantine Empire, which ruled the Eastern Mediterranean between 326 and 1453 -- is essentially "elitist," and that can not be helped. Scholars aren't pop artists: A good Byzantinist should read Turkish, Latin, Arabic, Greek and a dozen other languages. He should know the history of Cyprus, Syria, Greece, Turkey and Bulgaria and a dozen other lands. He ought to be familiar with diplomacy, archeology, architecture, military technology, theology and art. His work, by definition, is at least in part obscure.

The changes being wrought at Dumbarton Oaks may appear modest, but not to those who knew the place a few years ago.

Until old age drove her to her bed in 1967, Mrs. Robert Woods Bliss of Georgetown ruled Dumbarton Oaks -- that community of scholars with its gardens and its treasures -- like some stern wasp-waisted queen.

She had given the big house on the hill to Harvard at the start of World Warr II. But Harvard was far away, and Mildred Bliss was there. The teas that she held daily were part formal audience, part examination and part command performance. She would always wear white gloves and a hat. The Dumbarton Oaks Byzantinists she summoned to her teapot were less her employes than they were her subjects. She watched their manners closely.

It took more than erudition to please Mildred Bliss.She demanded "good deportment," and would say so in her will. "I charge those responsible for carrying forward the life at Dumbarton Oaks to be guided by the standards set there during the lifetime of my husband and me," she wrote.

It was a tall order. The Blisses gave their lives to Dumbarton Oaks. They bought the house, rebuilt it, installed its works of art, its Degas and El Grecos, its exhibition spaces, its garden paths and fountains. They owned the place for 20 years, lived in it for seven, and expected it to last.

That would require money, but the Blisses had a lot. Dumbarton Oaks today has an annual budget of $3 million and a staff of 90, yet costs Harvard not a penny. The generous endowment, thoughtfully invested in such stocks as IBM, now is worth perhaps $50 million.

Though primarily reserved for gentlemen and scholars, Dumbarton Oaks was given to the public, too. Mildred Bliss agreed that her 16-acre garden should be opened to the masses -- though not, of course, at mealtimes, or in mornings or in evenings, or in the heat of summer, when the elect of Dumbarton Oaks, the fellows and the staff, would be allowed, in privacy, to swim in the deep pool. But from 2 p.m. to 5 p.m., in spring and early fall, when the flowers were in bloom and the foliage most lovely, the public could be offered the garden she had wrought.

Mildred Bliss, red-haired, erect, would watch them as they wandered. She would carry her fringed parasol as a badge of rank.

Robert Woods Bliss died in 1962, his wife in 1969. Her ashes have been placed beside those of her husband in the wall that she prepared in the Dumbarton Oaks rose garden. Her inscription is austere. It states her name and dates. "It is touching, don't you think," said Georgetown's Joseph Alsop, "that her dates are wrong."

Alsop, the chairman of the advisory committee for the Dumbarton Oaks gardens, knew both the Blisses well. He enjoyed their conversation, "Mildred's whim of iron," and Mr.Bliss' yellowing pre-World War I champagne.

"The fortune was Mildred's. It came from Fletcher's Castoria -- 'Children Cry For It' -- the patent medicine. She was delicate, imperious, industrious and vain. Did you know that Mildred's mother married Bob Bliss' father? Bob Bliss and Mildred Barnes, though of course not blood relations, grew up as brother and sister. Mildred for some reason was embarrassed by the fact that she was the elder, and after they were married she refused to admit it. Even on her gravestone she lied about her age."

For many years Dumbarton Oaks seemed equally determined to resist the truths of time. The institution's first director, John S. Thatcher (1947 to 1949) was a close friend of the Blisses; the second, William R. Tyler (1969-1977) was their godson. Both appeared to do their best to keep Dumbarton Oaks exactly as they'd found it, maintaining the old Bliss traditions of hierarchy, privilege and "good deportment." Throughout the 1960s, and well into the '70s, Dumbarton Oaks seemed screened from the turmoil outside.

Of course, some had seen the dangers of isolation in the past. "As early as 1946," Constable has noted, "the director of studies warned that, 'A research institution in semi-isolation can develop dry-rot with surprising rapidity.'" And by 1977, when Constable arrived, Dumbarton Oaks seemed to be in danger both from outside and within.

He at once took steps to make Dumbarton Oaks appear less private -- a policy not without its pains. He began by opening the gardens to the public in July and August -- "the most controversial decision I've taken since I came" -- which denied the staff the afternoon pleasures of a private swimming pool. "To argue against the interests of the public is today most difficult," Constable says, "but the staff felt passionately about their pool, and I took some heat." Instead of building a new library, he began to search for space in the existing building. Eventually, he found enough for the next 16 years.

"You cannot understand Lebanon or Cyprus, or any other part of the Eastern Mediterranean, unless you know its history," says Constable. "We are not a monastery keeping the feeble light of Byzantinology burning in an age of barbarism. We should be, instead, a source of wisdom, and of inspiration. We are just beginning to open up our doors."

Robert Van Nice, 69, has an office in the basement. He speaks fluent Turkish, he's an architect, a surveyer, a historian and an artist. He is studying a monument. It took Anthemius of Tralles and Isidorus of Miletus less than five years to build the great church of St. Sophia, but they had slaves and armies. Van Nice, who understands 6th-century construction as well as any man alive, has, with interruptions, been studying their building inch-by-inch and stone-by-stone for more than 40 years.

Van Nice and his staff are publishing a book, or, to be precise, a portfolio of prints. Their complex, subtle drawings show every dome and stair, the masons' marks, graffiti, the door grooves in the floor. They are as beautiful as they are precise.

Justinian's great church is no ordinary building. St. Peter's is its only real competition. For 800 years St. Sophia in Istanbul was the largest building on the earth. All the other Byzantine churches that still stand in that city would fit into its nave. First a church and then a mosque, it was built, rebuilt, and built again; earthquakes more than once collapsed its mighty dome. Each repair and reconstruction can be read in Van Nice's drawings. "I fgiure," says John Wilson, one of the project's draftsmen, "that Bob has put on paper enough raw information for 200 Ph.D.s"

Donald E. Smith, the superintendent of gardens and grounds, stood beside a stout red oak on the lawn before the house. "I helped plant that tree," he said. "That was in 1952. I put my hands around it then. I couldn't get my arms around it now."

Smith was still a boy when he started pulling ragween in Bar Harbor, Maine, for Beatrix Farrand, the designed of the gardens. He knew both of the Blisses, he worked for Matthew Kearney, the garden superintendent from 1949 to 1973, and he has watched, with mixed emotions, as the times have changed. Shoe-proof flagstones have replaced the garden's grassy paths; 75 people once worked on the garden, now there are 13. "Mr. Kearney used to say that entering the house was like entering a temple. It's an institution now."

Smith remembers the elegance, the hauteur -- some would say the snobbishness -- that used to rule Dumbarton Oaks. "Things are different now. Today we're all supposed to feel that everyone is equal. Well, they're not," he said.