Only those who lived before the Revolution can know how sweet life can be.

Talleyrand

It was a period of gracious living, a time when women dressed extravagantly and spent a lot of time and money on their clothes. And it was a period when women were active and influential, strong personalities on their own who touched the politics, esthetics and economics of the day, often through their salons.

And it was an era done in by a scandal over a diamond necklace.

That was the 18th century and the women of that time are the focus of the new costume exhibit at the Metropolitan Museum of Art, "The Eighteenth-Century Woman," which opens to the public Dec. 16. It is the 10th, and in many ways the most remarkable, of the shows organized by Diana Vreeland, special consultant to the Costume Institute of the museum.

"The dresses worn tonight are no less splendid than the ones worn by rich women in the 18th century," commented Gerald van der Kent as he went through the reception line for the benefit preview at the museum last night. He should know. Van der Kent, the curator of Versailles, was one of the 650 guests at the $350-a-ticket event that is the major fund-raiser for the Costume Institute. More than a thousand additional guests, at $100 a ticket, were expected after dinner for the preview and dancing.

Marisa Berenson, dripping crystal beads and white Russian lynx, said she doesn't expect women to be tempted by the style of the clothes on exhibit. "I wore all those corseted dresses in 'Barry Lyndon' so I know what they are like and I can't imagine women wearing them by choice," she said. "But they certainly make you stand up straight."

Fashion royalty turned out in force to honor Vreeland and her show of gowns of 18th-century royalty. American designer Calvin Klein came with model Iman, Halston with at least a dozen Halstonettes including Bianca Jagger, Andy Warhol and Steve Rubell, and Bill Blass escorted Vreeland, who wore a black and gold Givenchy. Valentino, Gianni Versace, Gianfranco Ferre and hair stylist Sergio Valente were among the crowd from Italy, and Hubert de Givenchy from France. In that crowd it took Vicomtesse Jacqueline de Ribes, one of the world's best-dressed women, to stand out, and she did in Saint-Laurent black velvet and feathers.

"Who's that getting all the cameras," asked William F. Buckley, caught in the photo stampede rushing Raquel Welch, who was wearing a black and silver Norma Kamali. Carmen de Lavallade was wearing a red and gold brocade coat by her husband, actor/choreographer Geoffrey Holder, and Diana Ross was totally decked in feathers that she called "my own" when asked to identify the designer.

Robert McNamara, retired president of the World Bank, admitted this was his first fashion event but caught up with such old-timers as Henry and Nancy Kissinger (she in a Pauline Trigere), British Ambassador Nicholas Henderson and Lady Henderson, David and Susan Brinkley and Evangeline Bruce. "It's my 18th-century look," teased Bruce as she puffed up her draped skirt. She was wearing a pink flower on a ribbon to carry out the theme as well.

Following cocktails in the Englehard Court and dinner in the restaurant, turned into a modest chateau with trompe l'oeil French windows, guests had a chance to view the exhibit before winding up the evening dancing in the Temple of Dendur.

The exhibit goes far beyond the more than 100 featured garments. The daily life of the period is illustrated through paintings, furniture and art objects, plus painted fans, jewelry, laces and other accessories, tools of the coquettish ways of such extraordinary women as Madame Du Barry, Madame de Pompadour and Marie Antoinette. According to Vreeland, Marie Antoinette cared not a bit about being queen "but only wanted to be the best-dressed woman in France."

It includes the emerald "Liberty" necklace (the original emeralds replaced by paste) given to Benjamin Franklin by a Polish countess to help finance the American Revolution, and a reproduction of the "Queen's necklace," which, through great intrigue involving Marie Antoinette, helped spur the French Revolution. "The affair of the necklace is the preface of the Revolution," wrote Goethe.

"In the 18th century, everything was so on the move," said Vreeland. "Towns were becoming cities, life more of a community. People had to depend on one another instead of living in the wilds of the countryside in huge chateaus in small villages. Women came forward to plan for their own lives. They began to have ambition for themselves. Women went out and did what they wanted, as they do today."

The similarity between now and then was not the reason for the show at this time, Vreeland said. "I never calculate such things," she said. "It is only a coincidence. Mrs. Reagan likes beautiful things, attractive living. She likes the graciousness of life."

The clothes in the show are mostly from the museum's own collection and more than half of them are French, but almost none are traceable to Marie Antoinette or anyone else. "It's my greatest regret," said Vreeland, who has included the clearly identifiable wedding dress of Catherine the Great and several belongings of Peter the Great in past exhibits. "The rape and plunder of the last 250 years of western Europe has made it very hard to find things."

That loss is compensated for in the range of styles that display the luxurious fabrics of the time, and the way they are presented. Special mannequins were created by a Japanese sculptor to create the torso as it was shaped by the corset of the time, according to Stella Blum, curator of the Costume Institute. "How women must have suffered to achieve that look," she said. Most mannequins, even those from the Victorian period, have stomachs, while in the 18th century the ribs were pulled in, the stomachs flat, the breasts up and the spine rigid. An actual corset is on display on a mannequin sitting at her dressing table.

"The dresses and corsets were so constricting you could only raise your arms from the elbow," explained Blum. "That limited gesture with the arm is the reason for the exaggerated ruffle and embroidery on the sleeve."

The exhibit is not done in historical sequence, but according to how Vreeland thought the clothes looked best. She groups the prettiest dresses in a display against the popular pink color of the day, the glitzy dresses with gold and silver threads with a background of portraits, and another group against a blow-up of a drawing of the palace of Versailles.

Not only the styles of the time but also the exaggerations are exhibited in the show. An example is the wedding dress worn by Baroness Aelbrecht Von Slingelandt in 1759, with its huge panniers, side padding that required a special structure to be held up, and an elaborate hairdo, perhaps four feet tall, anchored to the ceiling during the exhibit.

Blum hardly expects anyone to start wearing panniers again, or such elaborate hairstyles, but the light, bright colors and the luxury fabrics will certainly catch the eye of today's designers, she says.

And even if people can't wear similar clothes today, the appeal will be extraordinary. "People love to fantasize," Blum said. "These clothes transport you to a romantic, elegant, never-never land. Even the rich don't look like the rich in the 18th century."

The exhibit, which has been funded by a grant from Merle Norman Cosmetics, will continue through August 1982.