STANDING IN a barny enclosure in one of the remoter recesses of the National Gallery of Art that passes for their studio, Gaillard Ravenel and Mark Leithauser are explaining how they designed the installation of 166 ancient and rare Cycladic art objects now on display in the East Building.

Ravenel, 38, is chief for designed and installation and Leithauser, 28, is his deputy. Together, they run a design department of 13 at the gallery, where as recently as a decade ago there was none, at least in any formal sense.

Their ambitious notions of how to stage successfully what gallery director J. Carter Brown calls "temporary exhibitions" are born of frustration with old-fashioned clinical modes of display that have been standard in most art museums for the better part of a century.

Traditional methods have not changed all that much in the display of paintings, still largely the turf of the curators, who once had the last word on virtually all displays. But with three-dimensional, objects the old methods of generalized and sometimes dim lighting, esoteric captioning and wall panels, bland background and haphazard organization, have proved wanting. "We've had to face the fact that displaying three-dimensional objects is much more difficult and more expensive than two-dimensional ones," observes Ravenel.

The solution first developed as regular policy under Thomas P. F. Hoving at the Metropolitan and later under Carter Brown here, is to build, in effect, a museum within a museum specifically tailored to the moods, the esthetics, and the historical values of the objects. Referring to the intimate, understated set of six sequential chambers designed for the Cycladic show, Ravenel says, "It's the sort of design we'd never done before and I doubt that we would ever do it again."

Neither Ravenel, who is a former curator and a trained art historian, nor Leithauser, who is a printmaker, was expert on the 5,000-year-old art of the Greek Cyclades Islands. "But we absorbed it fast, because it's all right there in that book," says Ravenel, tossing a huge German catalogue of an earlier show onto a studio table with a loud thump.

"Then we made a study of earlier exhibits. Mrs. Gourlandris [who with her husband, Greek shipping tycoon N. P. Gourlandris, owns the objects] came here to see Mark," Ravenel recalls, "and at the same time, since I was in Europe, I went on to Athens to see the works there."

The result of their study was not so much a guide of what to do as a litany of what not to do.

Leithauser pulls from a drawer a caption for one of the Cyladic objects: There is a minimal inscription in Greek followed by an English translation in parentheses."It was nice of them to send these along," he says, "but we deep-sixed them right off."

Ravenel walks over to a slide projector to flash on the wall shots of previous Cyladic-art displays. The first is a 600-piece survey at West Germany's Karlsruhe Gallery.

"They are large open rooms, with conventional glass boxes standing on metal legs," Ravenel points out. "The bases for the objects seem wrong. The Cycladic figures [many of them long, slender, female nudes that have been compared to those of Brancusi] are resting directly on the bases, so that in place of their great serenity the objects look like they are doing toe dances."

Then he shows a slide of the objects now at the Gallery, as displayed last year at Athens' Benaki Museum.

"Here they are all together in one big, light room," says Ravenel, "so that we see everything or anything, all at once without any focus. You just see too much, so it is harder to concentrate on anything in particular."

What one saw on the slides was hardly unattractive. In fact, it was considerably easier on the viewer than, say, the old-fashioned low, horizontal display cases in the front gallery of the Folger Library or in parts of the Library of Congress, over which one must stoop, and sometimes squint, to see the objects.

But the desire was not merely to make the Cycladic objects more presentable ("Like lots of archaeological things, they were downright unattractive lined up downstairs on the storage-room shelf," Leithauser recalls). They were to be displayed at their splendid best.

Brown recalls telling architect I. M. Pei when he was designing the East Building that the temporary display spaces should provide the flexibility for a different kind of show than the permanent-display areas. "I told him it should not be so cool. The viewer should be presented with an intense artistic experience" and should "have a clear sense of entering, observing in sequence and of leaving. There should be carefully determined pacing, textures, lighting and other variables.But in the process, the essential rubric is that the object has to come first. The risk is that the viewer will come away remembering the installation rather than the objects. We cast about in this area ourselves a bit and kept coming into the area of theatrical design.

"But that wasn't right because in the theater one is separated from the space and the object in it and so it is possible to cheat on stage by illusion. In museums, though, one is tied to the object itself and one must do all that's possible to help the viewer concentrate on its authenticity, its specifity."

So far as the Cycladic exhibit is concerned, Ravenel sought to overcome the errors of past shows by doing all he could to "simplify and clarify."

"Sometimes it was as simple as getting those figures off the bases and letting appear to float by the minimal armatures that Mark designed. The cases were scaled to the sizes of the objects," recalls Ravenel. "And we decided to have hexagon shaped cases along the walls, instead of flat-fronted ones so that there would be clearer views of the fronts as well as the profiles."

Since the works are now devoid of color, except for their natural marble, bronze or pottery pigments, it was decided that they would exhibit best in dark rooms against dark colors. Ravenel brought in "about 50 or 60 different fabric colors and tried out combinations of them with the objects under both incandescent and floresecnt lights." The choices were various combinations of dark blues, browns and reds.

The result is a show in which objects and setting are matched with unusual precision. "You don't often have the advantage of working at early stages with the real objects," says Ravenel, "and photographs are at best very imperfect guides to the color, textures and, most of all, the esthetic weight of an object."

The scheme also includes: maps of the islands, when they are revelant; photo-murals of them as they look today; diagrams of the range of possible object shapes; lucid wall panels on the cultural background; detailed background captions on each object, and soft, inconspicuous lighting, both general and spot.

Since Brown became the gallery's director about a decade ago, such shows have become standard and the design team is working - along with the large carpentry, silk-screening and conservation staffs - on several stages of different shows nearly all the time. They are now in the advanced stages of building a huge show called "The Art of the Pacific Islands" that will open July 1, also in the East Building. The objects (from about 80 sources) are in storage and the display area is nearing completion is about $400,000.

In height, at one point the Pacific art show comes close to testing the limits even of Pei's flexible spaces. Near the end will be a decorated front from a triangular New Guinea house that reaches 21 feet. Counting the setting, the display will reach considerably higher into the 40-foot clearance that Pei allows.

Pei believes the display areas are now the most versatile in any art gallery. "Some of the advantages of the East Building have been technically possible for some time," he says. "But no one had been able to afford them. The architectural, mechanical and electric facilities can be used in any combination. For instance, the floor supports either 300,000 or 350,000 pounds. I forget. You could bring huge Henry Moores right inside. No other museum could."

But if Brown can argue that the National Gallery is now "the pace-setter for the world" in such exhibits, such was not always the case. Several years before the gallery would start the design momentum that would lead to the Tut or Chinese or Dresden or Cycladic installations, the Metropolitan, under Hoving, was designing installations on a monumental scale.

Stuart Silver, Hoving's design director, recalls that the Metropolitan had been under fire for letting the blockbuster loan exhibits pass the museum by, so Hoving decided that he could debut with a blockbuster drawing on the Metropolitan's own enormous riches."In a a matter of weeks, we had to install almost 600 paintings, sculpture, costumes and objects. And the only thing they had in common was that they came from royalty. There was everything from master portraits to Marie Antoinette's dog house, and they had to be strung together by graphics, panels and visuals" created by Silver. It was the first time the designer recalls an art critic devoting an entire essay to the installation, as the New York Times' John Canaday did, praising Silver for "reconciling the seriousness of the museum with the exhilaration of the theater."

It was this element of theatricality that was eventually to start drawing fire to the Metropolitan's special exhibitions. The most criticized, the 1969 "Harlem on My Mind" show, was almost all show biz and no art objects. Even works by Harlem artists were excluded. Silver sounds relieved that it was not his design. "I just had to do a patch job on it" after the public outcry it roused, he says.

But in Silver's displays he came to tread a thin line between dos and Don'ts . "I always tried to make the art first and the spectacle second, but I went a little too far in one of my favorite shows, 'Before Cortez,' a pre-Colombian exhibit. We really pulled out all stops. And in retrospect, I think it was overstated. I had reached the saturation point on harsh lights and dark colors. And since then I've wanted to go for something softer." An example is the $860,000 traveling exhibit that Silver has designed for the Folger's most valuable artifacts, which will be shown over the next two years.

The National Gallery has drawn less criticism for over-producing. But at least two shows, the one on the works of landscape designer Frederick Law Olmstead and the Bicentennial show, "The Eye of Thomas Jefferson," are described as "flops" by a gallery official who asked not to be named, a judgment in which critics concurred. In both cases the installation overwhelmed the objects. (Neither was a Ravenel show.)

No other institutions can afford the museum-within-a-museum concept on the scale of the Metropolitan or the National Gallery. Val Lewton, the designer for the National Collection of Fine Arts, points to the newly shaped space that houses the new show called "Western Art": "Anytime I put in dry walls for a show like this," he says, "they ask me if it can work for the next show or the show after that, so that we can save. On the other hand, though, having less to work with may sometimes mean that we stick harder to the basic display philosophy that 'more is less.' " A painter himself - like the Hirshhorn's Joe Shannon - Newton gets involved in hanging paintings. "I've moved some of them around in the Western show and when the curators return from out of town they'll probably raise hell, and we'll work it out."

Other Washington museums avoid elaborate installations, partly for financial reasons. And the Corcoran's assistant director, Jane Livingston, points out that another reason is that "we have this great natural resource: acres of natural light."

As for the Phillips Collection, Ravenel makes the point for them. "What was more exciting than the days when Duncan Phillips was moving the paintings around all the time," he recalls, "and you would go there and he would have the paintings seeming to literally be talking to each other!" CAPTION: Picture 1, "Displaying three-dimensional objects is much more difficult and more expensive than two-dimensional ones," says Gaillard Ravenel; Picture 2, He and his colleague, Mark Leithauser designed the Cycladic Art show at the National Gallery; Pictures 3 and 4; they check the finished Cycladic exhibit, the floor plain for before going over their plans for the Gallery's upcoming "The Art of the Pacific Islands" display. Photographs by Douglas Chevalier - The Washington Post