Scott Barrie doesn't make a secret of his formula for designing. After clothes have been skinny for a while he starts cutting them very full. After they have been simple in design, almost "poor" looking, he switches to opulence.

He's hardly alone with this scheme pected about fashion all along. What puts Barrie apart from other designers is his skill with matte jersey and his knack for making clothes, skinny or full, in matte jersey or other fabrics, very soft and sexy.

Yesterday Barrie, who is now in his "full" period, was at Bloomingdale's. "You wouldn't go out in that without a slip, would you?" whispered a gray-haired woman to Barrie during the early morning fashion show as a model passed by in a printed georgatte full-skirted dress, her legs clearly silhouetted underneath.

"If it makes you more comfortable, you should wear a slip," came the answer. But to Barrie, the fullness of the dress provides enough coverage for customers like Jacqueline Onassis and Lauren Bacall. "Besides, when I put separate slips with the dresses, they sometimes get lost in the store," he explained to another customer.

Barrie, whose clothes are carried also by Saks Fifth Avenue and Elizabeth Arden, doesn't hesitate to try his hand at fashion fads.Last fall he tested mini-tunics over tight pants and found, "It only confused the customer about lengths and sent her back to pants."

And after a try at tapered pants he concluded, "My customers simply prefer a wider leg."

For summer, he's varying the silhouette slightly by giving many of the designs tight, marked waistlines. But big changes will come for fall, he says.

"I think women are ready for a little opulence," said the designing a line of scarves, handbags, and menswear for fall.

Barrie says he's tried of matte jersey now, though he was one of the first to use it in American 10 years ago when he could only buy it from fabric companies that made it to be used in turbans. And he's "bored" with silkcrepe de chine, the lightweight textured fabric that has been particularly popular in the past year. "It's overused and overdone," he says,

Barrie has in ming big brocade skirts like those worn in the Russian ballet and colorful printed tulle designs (a silk and rayon netting.) "It's hard to print," he pointed out. "The dye keeps going through the holes."

Barrie senses that women are ready for richer fabrics, judging by their response to the gold lames at the increasingly popular gold boots and gold shopping bags from Fiorucci. Besides, the Russian costume exhibition of opulent garments at New York's Metropolitan Museum has proven the museum's most popular costume show to date, and the books on Russian costume are selling are selling well.

"I don't think women want to look "Poor" any more," says Barrie.