THE SECOND Empire. Art in France Under Napolean III," the wry, enormous loan show at the Philadelphia Museum of Art, is deliciously subversive. Frivolous, luxurious, it pokes a hundred holes in that agreed-on fable called The Origin of Modern Art.

Conventional wisdom has it that modern art was born - in a thrilling confrontation - in mid-19th century France. The story has been told in countless college lecture halls, text-books and exhibits. The protagonists, by now, are thoroughly familiar. And we all know who wins.

On one said stands the Good Guy, most likely an Impressionist, wearing a white beret. The Establishment reviles him and his garret is unheated, but his strength is as the strength of 10 because his art is New.

His opponent - the Official Artist trained in the academy - twirls a black moustache. His art is highfalutin, his paintings put on airs. He fawns on the aristocrats, sneers at the bohemians and schemes to keep The Daring out of the Salon. In most tellings of the tale he doesn't stand a chance.

"The Second Empire" is, however, a revisionist exhibit. Instead of paying rote homage to esthetic revolution, it stars the officials artists in vogue at the time. Their overdone, encrusted art long has been anathema to modernism's minious, and it is still not quite respectable. The popular, officials artists of the Second Empire were not afraid of imitation. Their work is often pompous, their taste is often laughable, but their craftsmanship is glorious.

The Second Empire began in December 1852, when Louis-Napoleon Bonaparte, a nephew of The Little Corporal, was proclaimed Napolean III, Emperor of France. It ended in September 1870, when he surrendered to the Prussians on the field of battle. The British still applaud Victoria, his contemporary. The fickle French, however: do not celebrate their last Emperor's reign.

"This is a period," writes French scholar Jean-Marie Moulin in the 464-page exhibition catalogue, "that has been ignored - one might almost say erased - by French art historians, who have considered it insignificant, if not monstrous."

Second Empire Paris must have been a lot of fun. It was a time of soaring confidence, splendor and frivolity. The music halls were full, as were the theaters and the Opera. Escoffier, "the king of cooks, the cook of kings," was mastering his art. Second Empire engineers opened the Suez Canal and changed the look of Paris. Busy Baron Haussmann was ramming his broad avenues through the center of the city. The Opera was being built, so were the Gare du Nord, Les Halles, the central market and the Bibliotheque Imperiale.

Second Empire art is extravagant, unfettered. Imagine a huge table heaped with richest pastries made of sugar, chocolate, cream and butter by an army of great chefs. Those whose eyes have dieted on minimal art, on tables made of chrome and glass, on Bauhaus Good Design, may find this show excessive. It overfeeds the mind.

Ornament is everywhere. The facades of Second Empire buildings as covered with pediments and statues and interwoven references to the buildings of the past. Fronds and vines and tendrils crawl in rich profusion over Second Empire porcelains and glass. The metal work displayed is modeled, chased, engraved, or set with precious gems. Cherubin and putti swarm on Second Empire candlesticks and clocks. Every inch of every leg of every chair and table seems to be encrusted with veneers, inlays and carvings. There must be a thousand nymphs and goddness, most barebreasted and plump, showing off their charms in the objects on display.

This art does not pretend to be dashingly original: on the contrary, it throbs with affection for the past. It months pay homage, sometimes simultaneously, to Greece, Pompei and Rome, to the Gothic and the Romanesque, to the Near East and the Orient-Bedouim on horseback, Romans wearing togas, satyrs, driads, saints, run riot on the surfaces of these costly crafted things.

Many of them seem intentionally, if only partially, old-fashioned. But underlying their eclecticism is a touching, often overlooked, confidence in progress. The official artists of the period, though conservative in taste, innovated radically in technique.

The pavilions of Les Halles, though decorated here and there with rosettes and curlicues, were made of iron and glass. The Rattle of the Prime Imperial, made in 1856, includes a whistle for the baby and emeralds and diamonds, no doubt for his parents. This extraordinary object also boasts four statuettes and two small medallions - all made of aluminum - and may be the first work of art in which that new metal was used.

The painting on the cover of the catalogue accompanying the show is not a painting after all, but a piece of wallpaper whose fabrication required thousands of separate printing plates.

Despite the grand pretensions of its rulers, the Second Empire in France was dominated by the rising bourgeoisie. The newly wealthy French enthusiastically approved the news, old-fashioned, fashions. Many of the more ornate objects in this show were not made by solo craftsmen, but by large, efficient factories that busily produced geegaws of all sorts for the prospering middle class.

Cabinet maker Charles-Guillaume Diehl, whose "Merovingian" medal cabinet, borrowed from the Louvre, is a highlight of the show, employed 600 workers. Together they produced "necessaries, liquor cabinets, game boxes, tea caddies, boxes for gloves, shawls, pins . . . thuja pieces, fancy items with bronzes and porcelains, drawing room tables, ladies work tables," and who knows what else. In 1867, the population of Besancon was approximately 50,000 of these 15,000 were employed in workshops decorating watches manufactured by the Swiss. In 1866, the watchmakers of Besancon produced 305,435 watches, of which 101,309 were gold.

Much Second Empire art, with its pseudo-Gothic, pseudo-Roman, pseudo-Renaissance designs, might seem offensively bourgeois, peculiarly phony, but its charm is underiable. Bubbling within it is an unquenchable enthusiasm, a willingness to please. Second Empire buyers could not resist such art. "Much later," writes Moulin, "the 'enligtened' sector of the same bourgeoisie, confident in its own continuity, called this art 'pretentious' (pompier) and 'official,' and willingly ridiculed it, forgetting that this art sprung from within its own ranks. The Second Empire was the naive phases of bourgeois art."

Its painters, however, were not primitive at all - at least not in technique. They learned a lot in their academies. It is true that one soon tires of all those half-draped women, all those noisy battle scenes, but there is no doubt at all that they knew how to paint.

Most art texts insist that Degas, Rodin, Manet, Courbet and other early modernists were implacably at war with the imitative backs of French official art. But the history of art is rarely so clear-cut. The reactionary artists of the day - and their supposed enemies. Degas and Monent, for instance - here hang side by side. One virtue of this show is the witty way it shows us that the paintings of the old guards, and those of the young Turks, were in spirit much alike.

Gustave Courbet, for instance, is represented here by his "Woman With a Parrot." Clearly she belongs with all the other languid and seductive nudes portrayed in the Salons. Delacroix and Ingres, too, are prespresented by large, crowd-pleasing pictures of women without clothes. Both of these masters are today in high repute, though Delacroix here shows us swimming nymphs, and swans, a rape, a burning castle. "The Source," the lovely Ingres borrowed from the Louvre, is as sexy as a centerfold. One Degas on view is "The Daughter of Jephthah," ahuge, tale-telling Bible scene that is wholly of its time. "Young Girl With Roses on Her Hat," the Rodin terracotta, shows an innocent young lady, "wide eyed, pert-nosed, with a bee-stung mouth." She wears a fashionable hat. The message of this bust - and of a hundred other works here - is the prettiness of its model, not the fight for modern art.

"The Second Empire," with all its early photographs, duelling pistols, jewels, wallpaper and armchairs, fabrics, clocks and vases, is a show whose time has come. We do art a great disservice when we lie about its heritage, exaggerate its newness. Rodin and Degas, Monet and Manet need not be seen as warring with official taste, much as that might comfort us. This exhibit shows us that their supposed enemies were their friends and allies, too.

Joseph Rishel and Kathryn Hiesinger, the two Philadelphia Museum curators who conceived the exhibition, received much help from French scholars and collections. Three-quarters of the works on view have never in the past been seen out of France. The show will travel to Detroit - and, in 1979, to the Grand Palais in Paris - after closing in Philadelphia on Nov. 26.