BEFORE YOU can talk about the new age dawning at Dumbarton Oaks, you have to figure out what Dumbarton Oaks exactly is.

Yes, it's that big house on R Street in Georgetown with the pebble driveway and the wrought-iron fence. Yes, it's one of the world's great gardens, 10 acres of organized flowers and trees, pools, fountains, arbors and gazebos, a little world hidden behind a brick wall and only recently opened daily to the public. And yes, it's a museum of Byzantine art with the largest Byzantine library anywhere, and it's also a museum of pre-Columbian art, and there are resident scholars who get to swim in the pool, and something about symposiums and concerts and grants and don't forget the library of garden history and landscape architecture . . .

But what is it really and finally? Or, for heaven's sake: What is it most?

"Frankly, I doubt whether we'll ever pull it all together in some new unity," says director Giles Constable. "What we have here is always diverse, according to the wishes of the founders, and it would be Mickey Mouse and artificial to try and impose a single purpose."

The building itself, he says, and its location atop the Georgetown hill, is the real unity of Dumbarton Oaks.

Sometimes he asks the fellows what they think is the most valuable single work of art on the place: the El Greco, the Icon of St. Peter, which cost a Picasso and a Matisse, or what. He always concludes that the crown jewel is in fact the garden itself.

"What goes on here will ebb and flow according to what the scholars are interested in, and there will be two doing this project and one doing that and another doing something else, and we don't want the Procrustes bed of a single conception here. We're leery of central themes."

In a word, there is no pigeonhole for Dumbarton Oaks. It is what it is, and one might as well accept its many faces and learn to know them one by one.

It is getting easier to do so these days, and this is our subject, and none too soon.

Before the 54-year-old Constable arrived on the scene, Dumbarton Oaks had a distinct reputation for being private, exclusive and social. Owned by Harvard, you know. It used to close all summer because that was when the Blisses left town. ("Doesn't everybody?" Mrs. Bliss is supposed to have asked in innocent astonishment when someone suggested leaving it open.) The atmosphere at Dumbarton very much reflected its owners, their tastes, their interests, their personalities.

Robert Woods Bliss, a Foreign Service veteran and former ambassador to Argentina, and Mildred Barnes Bliss, heiress to the Fletcher's Castoria patent medicine fortune, bought the property in 1920. It had been a working farm, 53 acres worth of a land grant acquired in 1702 by a farmer from Dumbarton-on-the-Clyde in Scotland. The Georgian house was built in 1801, the adjacent orangery in 1805.

The Blisses soon put their mark on the place: Within two years Mildred Bliss had landscape designer Beatrix Jones Farrand working on the gardens, a project still under way when the whole 16 acres of grounds and buildings were donated to Harvard University in 1940. The Music Room was built in 1929. The superb eight-faceted wing for the pre-Columbian museum was finished in 1963, designed by Philip Johnson, and the same year another wing was added for the Garden Library.

And all this time, until she took to her bed in 1967, Mrs. Bliss could be seen roaming the garden under her fringed parasol, presiding over teas in white gloves and hat, checking that the scholars were jacketed and necktied at dinner. She decreed that the gardens would be open to the public from 2 to 5 p.m. in spring and fall but that otherwise the place was to remain basically a private residence (until she moved down the street in 1940), and after that a home for the scholars. Robert Bliss died in 1962 and his wife seven years later.

"Dumbarton Oaks went through a slightly difficult period in the late '60s and early '70s," says Constable. "It was partly what was happening to everybody, but it was also that decisions were being made in a different way. That rather paternalistic way the Blisses had: People would take things from them that they probably wouldn't take from me, or from a board. We became institutionalized after they died."

Interestingly, after a period of uncertainty about direction, including a famous controversy over a proposed library on the north vista, Dumbarton Oaks is returning to original principles in some ways. In the rose garden, for instance:

"Over the years the various beds tended to become each a different color. But Mrs. Bliss in her original plantings had intended a sweep of colors across the whole garden, from yellows and whites gradually to pinks and salmons, and then reds toward the south. This had subtly vanished over the years--it was nobody's fault--but now we've taken out the best part of 1,000 roses and replanted."

It is in the new, however, far more than any revival of the old, that Dumbarton Oaks shines today.

The gardens are open all year now, except on holidays and rainy days, from 2 to 5 p.m. and for an extra hour April through October, when a $1 admission is charged. (There was far less outcry over this fee than over Constable's move to open the grounds in July and August his first year here.) A $10 season pass is also available.

"The dollar doesn't bother 'em," he says. "We think more people than ever are coming in, though we never count. We've always been lucky about litter and casual damage: We tend to the carriage trade because of where we're located, but we would be happy to have a great many more people who are not the carriage trade and who might leave an occasional can or bottle. Picking flowers isn't a problem either. I think the very beauty of the place imposes itself on people in a nice way. Actually, the greatest danger is to the lawns, through the sheer amount of walking, high heels and all that."

Sometimes areas are rather diffidently stringed (not roped) off when threatened. Years ago, even the gravel paths were grass. The most visible damage site at Dumbarton is the Palladian orangery, which has been attacked by carpenter bees and rot. Beams are being replaced and a slate roof will be added as part of a $400,000 repair job.

Constable would love to install an authentic Byzantine garden, complete with falling water, if he and Donald Smith, the longtime superintendent of gardens, could find a place for it. Incidentally, he has had printed Mrs. Bliss's typescript flower book and key to the gardens.

The biggest changes at Dumbarton Oaks may not wave in the breeze and change color with the seasons, but they are plenty visible nevertheless. The Byzantine icon show and "Gardens of the Middle Ages" exhibit, both built around symposiums that were open to the public, have just ended. The concerts and lectures are drawing more people than ever on a first-come, first-served basis. This fall a major show and symposium on Aztec art will be held by Dumbarton Oaks in cooperation with the National Gallery of Art.

"We hope to open lots of people's eyes to the quality of this art," Constable says. "This is not primitive by any standard. It is vast and varied, yet possibly the least appreciated of all the pre-Columbian work. So often it is treated as archeology or shown at the Museum of Natural History, that sort of thing. When Mr. Bliss began collecting it before World War I it was all in ethnographic museums as curiosities. He designed his collection to show its esthetic value. I mean, this is art quite comparable to anything in the Oriental or European art traditions."

Next year's fellows will fit right into this picture. As a pioneer in translating Mayan hieroglyphics, Dumbarton unhesitatingly picked David Stewart as one of its two dozen fellows. Stewart has been a publishing scholar in this field for three years. He is 17.

"He didn't qualify as a junior fellow, because they're supposed to be working for an advanced degree and he was still in high school. So we made him a senior fellow, which is based on qualification by comparable work. He will live at home, luckily. We're not really equipped to deal with that aspect."

You can't say Dumbarton Oaks isn't flexible. The oldest fellow, by the way, is 81. Most of them now live in a building just down the street from the mansion. They no longer have to wear jackets to dinner.

The program remains resolutely small and scattered, on the model of city-states rather than an empire. It refuses to concentrate its funds on a few huge directed projects; it prefers to let scholars follow their own instincts under the guidance of their senior colleagues. Over the years expeditions have gone to Istanbul, Cyprus, Syria, Italy, Tunisia and other exotic places. One scholar has gone to Jordan to investigate the boundaries of the Roman empire. Others have been photographing or drawing ancient mosaics from San Marco in Venice to St. Sophia in Istanbul.

Over half the Dumbarton Oaks budget of nearly $4 million goes to salaries, including 90 full-timers. The recession forced out most of the full-time editors (Dumbarton Oaks publishes about 10 books a year) in favor of free-lancers; there are more volunteers and a docent program; the big field projects have been cut back, as have acquisitions, though Constable insists that Dumbarton Oaks will continue to "look at the totality of material remains and scholarship" and will not become simply a library.

As he says, "An ivory is a book that can be read. Fewer people than I would like treat the collection as a great research source. But then, how many museums really have research staffs today? They become increasingly places of collection and display."

Giles Constable seems to have had an appointment in Samarra for this job. Born in London, a descendant of the landscape painter, he grew up at Harvard, where his father was a scholar.

"I had my first job when I was 11, mending pots in the basement of the Fogg Art Museum. Then I was a stack boy in Widener Library."

He got his B.A. at Harvard in 1950, his doctorate seven years later, taught medieval history at Iowa for three years before returning to Harvard for a 19-year professorial career. Long interested in Dumbarton and its Byzantine library, he served on various Harvard committees for it, gradually became more and more involved with it and finally was named director.

"For better or for worse," he says, "I stood for having a scholar running the show. We scholars ought to be able to do our own thing, I think."

A 12th-century specialist with a Guggenheim and several books (including "The Letters of Peter the Venerable" and a study of monastic tithes) under his belt, he finds he has almost no time for his own work now, aside from the occasional course taught at area universities.

"If I ever do reach the point where I begin to resent the fellows, whose work I facilitate without doing it myself, that'll be the time for me to go. But it hasn't happened so far. Meanwhile, being a scholar myself, I think I know their strengths and weaknesses."

Even as a scholar studies the past, however, he hopes also to enlighten the future, to help provide insights into the Middle East and South America and other subjects of concern at Dumbarton Oaks with a vigor and a concern for the public that wasn't especially evident there 30 years ago.

Oddly enough, he feels sure the Blisses would have wanted it that way. "We have opened up in ways they probably wouldn't have . . . then," he says. "But if they were here today I think they would have seen things differently."