you may have guessed -- an exhibition of magnificence. "The Treasure Houses of Britain: Five Hundred Years of Private Patronage and Art Collecting," which goes on view today in the National Gallery's East Building, though dented by unseemly hype, was meant to awe. And does.

It has a head of Aphrodite that may be by Praxiteles, and armor worn in jousting, and Henry VII's psalter; it has the "Rainbow Portrait" of Queen Elizabeth I dressed in wondrous splendor; it has a settee made of antlers, and a wine cooler of silver big enough to bathe in, and banners won at Waterloo, and a 14th-century sword. Yet the awe the show engenders is oddly insubstantial. Its Titians and tiaras, Claudes and Canalettos, Chippendale chairs, bowls of gold and silver, arms of war and beds of state, porcelains and marbles -- those 835 privately owned objects, one grander than the next -- suggest the not-quite-real, the cleansed, the incomplete.

The show enlists a dream.

It is in part a dream of romance dreamt by those Americans enthralled by aristocracy, and in part a dream of power dreamt by the aristocrats. Commoners exploring these treasure-laden galleries may find themselves in reverie among the nobles of Great Britain -- much as the nobility, with comparable innocence, forgiveness and nostalgia, once dreamed themselves companions of the Knights of the Round Table and the governors of Greece.

John, Lord Lumley, who in 1580 commissioned that life-sized wooden horseman, was among the first, though surely not the last, noblemen of England obsessed with medieval chivalry and "the vanished glories of his house." John, 5th Earl of Exeter, who died in 1700 (of a surfeit of fruit), had himself portrayed in marble as a senator of Rome. In 1822, to celebrate the victories of the Duke of Wellington, John Flaxman made a massive silver-gilt "Shield of Achilles." "Love Among the Ruins," painted by Burne-Jones in 1894, hymns Italy, the Renaissance and Arthurian romance.

Standing in the corner of one colonnaded gallery is an urn of polished silver. When sent in 1815 by Lord Byron to Sir Walter Scott, it contained the bones of long-dead heroes "found in certain ancient sepulchres within the long walls of Athens."

One feels, throughout this show, a yearning for the grandeur of the past.

As a venture into scholarship, and into museology, the country house exhibit could not be much improved. No museum anywhere has done a show so sumptuous, enormous or expensive. Hurrah for its evocative, meticulous display. Were it only to be greeted as a survey of art patronage, a mere gathering of valuables, or just a history of taste, this show could not be faulted. But it evokes something more. It conjures up the country house, but just part of it, and in doing so misleads.

Great Britain's country houses were not really like this, even in their heyday, and are not like this now. Despite their rooms of state, and their 60-bedroom scale, they were not palaces but homes. They were buildings people lived in, their plumbing was imperfect, they smelled of damp and dogs and candle smoke, and of human labor. These objects impress; some intentionally intimidate. The stately homes evoked here seem to have been built as places where people could be rich together. But poor folk lived there, too.

As you wander through this layered show, take a moment to remember the vast, and vastly underpaid, household staffs who polished all this silver, lit the candles in these sconces, and then hid themselves away in meager rooms beyond the green baize door.

They were sometimes treated horribly. "At Welbeck," writes Mark Girouard in "Life in the English Country House," "the Duke of Portland (admittedly eccentric if not mad) sacked any housemaid who had the misfortune to meet him in the corridors. Housemaids in a country house in Suffolk had to flatten themselves face to the wall when they saw family or guests coming. In Wiltshire an anonymous Lord M., as reported by his footman and valet, 'never spoke to an indoor servant except to give an order and all the ten years I was with him he never, except on Christmas and New Years' days, gave me any kind of greeting.' "

Only at the show's end, in a wall of telling photographs (and in a Chippendale doll house, which does include a kitchen), are all those nameless ladys' maids and laundry maids, first and second footmen, stable lads and butlers, halfheartedly acknowledged. The gallery's exhibit, as such shows tend to do, congratulates the wealthy. And ignores the poor.

(There is one exception, a 1670 canvas called "The Tichborne Dole." It depicts a ceremony still enacted annually at Tichborne Park: "Tradition has it that in the 13th century Lady Mabella Tichborne on her deathbed asked her husband to grant her the means of setting up a charitable bequest . . . He agreed to give for this purpose the corn from all the land his wife could crawl round while a brand was burning. In her weakened state Lady Mabella managed to crawl around a field of 23 acres still known as 'The Crawls.' ")

For most of the five centuries surveyed by this show, the British rich were richer than we can now imagine. Where did their money come from? How did they hold their power? Such questions are not stressed here. Imagine an exhibit that deals with Mount Vernon or Monticello, yet does not speak of slavery. This is such a show.

"In 1873 (the first occasion for which accurate records are available)," writes Girouard, "four-fifths of the acreage of Great Britain (including Scotland and Wales) was owned by less than 7,000 people, the great majority of whom were country house owners. At a local level, they dominated the countryside. They were equally entrenched in national government. By the 18th century, they provided at least 80 percent of the members of the House of Commons and virtually the entire House of Lords."

Perhaps it is no wonder that Britain's country houses were long viewed with resentment -- think of all the snobbery and the gathered wealth that they still imply. Once their power weakened, an unforgiving nation made them pay and pay. Until very recently, death duties and rates were applied so severely by successive British governments that many country houses were destroyed, most of them by owners who could not pay their taxes.

Between 1875 and 1975, 1,116 country houses were demolished.

In 1955, their owners tore them down at the awful rate of one every five days.

The present show, in many ways, is a plea for help. Though hostility has lessened -- the nation has begun to sense how much it has lost -- the British country house, that great aggregate museum, is not out of danger yet.

Its libraries and pictures are still being dispersed, and not only to the Getty Museum, but to England's art museums, too. Taxes are still killing, old plaster walls are cracking, roof timbers are rotting, and gardeners who once trimmed the privet for five pounds a year now earn five pounds an hour.

The big house on the hill is not what it once was. The hill has been eroded. The house has been marooned.

When you wander through this awesome show, look beyond its grandeur and think of the decay. You are seeing a veneer.

No single country house contains works as well selected as those here on view.

The four men most responsible for the exhibition -- Gervase Jackson-Stops, architectural adviser to the National Trust in London, Gaillard F. Ravenel and Mark Leithauser of the gallery's design staff, and of course J. Carter Brown, the gallery's director -- must have seen more than 200 houses and who knows how many objects, surely tens of thousands, before they picked the splendid few exhibited.

They have done a favor for the houses represented. They know beauty when they see it. They have, or so it seems to me, a finer eye for quality than did many of the nobles who bought the works displayed.

Many British country houses are stuffed with florid things. Many British nobles (Lord Lumley, Lord Egremont, the Earl of Arundel and Lord Burlington are among the exceptions) knew more about their roses, their guns and hounds and horses, than they did about their art.

"Their taste," writes Lord David Cecil, whose high breeding lends him credence, "was a little Philistine. Aristocratic taste nearly always is. Those whose ordinary course of life is splendid and satisfying find it hard to recognize the deeper values of the exercises of the solitary imagination; art to them is not the fulfillment of the soul, but an ornamental appendage to existence. Moreover, the English nobility were too much occupied with practical affairs to achieve the fullest intellectual life. They admired what was elegant, sumptuous and easy to understand: portraits that were good likenesses and pleasing decorations, architecture which appropriately housed a stately life."

A sense of gaudiness avoided, occasionally just barely, is felt often in this show. Looking at these painted plates, tureens shaped like chickens, silver cups and gilded chairs, one cannot help recalling Citizen Kane's Xanadu, and mansions built in Hollywood, and overpriced hotels. Objects of this sort, if imitated coarsely, as they have been incessantly on both sides of the Atlantic, epitomize bad taste.

The curators deserve much praise for their editing. That tangle of esthetics, that mix of good and awful, baroque and neoclassical, of very old and rather new, sensed in most country houses, has been brilliantly unsnarled by their installation. Their 18-room exhibit has four chapters: "From Castle to Country House: 1485-1714," "The Grand Tour: 1714-1770," "The Gentleman Collector: 1770-1830," "The Romantic Vision: 1830-1985," and an epilogue. The period rooms they have designed are not recreations, but rather evocations. In almost every gallery there are masterworks displayed.

It might take a while to see them. Not all of them are large. Look, for instance, at Isaac Oliver's 16th-century miniatures. Three may be discovered beside Lord Lumley's horseman in the room of stone with which the show begins. One, two inches high, shows an unknown man consumed by flames of passion. The second, slightly larger, shows the Brothers Browne, all dressed in black satin. Its companion, which portrays a knight reclining in a wooded glade, is Shakespearean in spirit. It might portray a scene from "A Midsummer Night's Dream."

The finest works of England's art may well be her portraits. They confront the viewer boldly throughout this grandly peopled show. A number are enormous. The largest portrait hanging in the Jacobean Long Gallery, whose windows are of leaded glass, whose ceiling has been timbered, was painted circa 1610. It shows a life-sized Prince of Wales mounted on a proud white horse and leading Father Time. Another splendid portrait, smaller but just as grand, is displayed in the next room. Painted by Samuel Cooper, circa 1650, it shows Oliver Cromwell, portrayed as he wished, "warts and all."

When Cromwell's revolution failed, Restoration England responded with new confidence to Continental art, first to eye-fooling interiors and Delft pottery from Holland, and later to to the unashamed effulgences of the French Baroque.

The best of British Baroque art has, despite its curlicues, a direct English bluntness. Perhaps the best example is Grinling Gibbons' still life. Though its materials are not precious -- it is made of oak and limewood -- it is among the among the grandest objects in the show. Young Gibbons was discovered on Jan. 18, 1671, when the diarist John Evelyn discovered him carving a copy of a Tintoretto crucifixion in "a poore solitary thatched house in a field." Within weeks the carver had been presented to the king. He is represented here by a wonderous life-size carving of scallop shells and lobsters and fresh-killed birds and fish.

Rare is the collector who has almost single-handedly changed a nation's taste. One such man was Richard Boyle, 3rd Earl of Burlington and 4th Earl of Cork (1694-1753), who preferred the calm and classical style of Palladio to the twists of the Baroque. Burlington liked quiet, stately rooms of "harmonic proportions," cubes and double cubes, decorated grandly with Italian art, and for most of the next century England followed where he led, to Mediterranean classicism, to Venice, Florence, Rome.

"Sir," Dr. Johnson said to Boswell, "a man who has not been to Italy is always conscious of an inferiority, from his not having seen what it is expected a man should see."

The aristocrats of Britain went to Italy in droves, and often in high style. Brinsley Ford reports that "William Beckford traveled in such style, with three carriages for his retinue, with outriders and relays of spare horses, that he was mistaken for the emperor of Austria traveling incognito and was honored with an imperial ball." It was there they bought their Titians and their marbles, too. Washington has rarely seen antique sculpture as impressive as the Greek and Roman pieces in the sky-lit Sculpture Rotunda that was specially constructed for the present show.

Grand Italian paintings and white antique marbles are fine for rooms of state, but the British never lost their love for art that was more intimate. Hence the fine Dutch pictures here. Of these the most delightful may be Jan van der Heyden's little view of Amsterdam. It hangs beside a Rembrandt and more than holds its own.

The country house has always been rooted in the land. The woodlands, parks and fields that made country life worth living are suggested by the paintings in a pair of splendid galleries. The first is filled with landscapes by Poussin, Ruysdael and Claude. The second is devoted to the hunt and to the horse.

The show might have included a gun room and a tack room. In their absence Stubbs will do. His paintings have a clarity, a strange time-stopping stillness that has been rarely equaled. Two of his grand pictures here, "A Water Spaniel" (1804) and "Mares and Foals Without a Background" (1762), by themselves would justify a visit to this show.

It closes with a burst of high imperial grandeur. The Waterloo Gallery contains imposing pictures by Rubens and Velasquez, and Canova's chaste-but-sexy statue of the Graces for which the sixth Duke of Bedford built a little temple. Also on display there are Constable's "The Lock," and Raeburn's nicely moody portait of Sir Walter Scott, which leads to the chapter, the one devoted to romance.

Scott's long, romantic novels led legions of Victorians, and the queen herself, to the Scottish highlands, and the Landseers here are shown against Black Watch Tartan walls. The Pre-Raphaelite works that follow, by Burne-Jones and Alma-Tadema, are hanging on William Morris wallpaper.

Next come Edwardian pictures -- of still-touching confidence -- by Sargent and by Lavery. Their images of elegance, of awesome wealth and endless teas, remind us that an era was about to end.

The show ends with a moan, with undistinguished little sculptures by Epstein and by Henry Moore. In this context they're pathetic. They make us think with wonder of all that went before.

The country house exhibit deserves repeated visits, one, say, for the portraits, a second for the furniture, a third for just the silver, or the horses and the dogs. Its paintings are exceptional. Scattered through its galleries are little one-man shows of Oliver and Zoffany, Stubbs and Wright of Derby, Canaletto, Landseer and Van Dyck.

The exhibit was supported by a grant in excess of $1.5 million from the Ford Motor Co. Ford also paid for the catalogue (which is thicker than the phone book) and for the useful guide book that, in lieu of lengthy labels, will be given to all visitors. The show will not travel. It closes March 16.