"Let's try it over there by the rosebushes," said Secretary of Defense Caspar Weinberger, resplendently Victorian-looking in white trousers, a dark jacket and a straw hat with a red-and-white-striped hatband. He escorted the Victorian-clad Mrs. Weinberger to a secluded corner of the British Embassy's rose garden, brandished a very non-Victorian 35mm camera and proceeded to take her picture against a rosy background right out of a Trollope novel.

British Ambassador Nicholas Henderson, the host of last evening's annual Folger Library benefit picnic, had a Dickensian air, with a cane and a yellow vest as well as the basic Victorian costume sported by Weinberger, who was co-chairman of the picnic with his wife Jane. Henderson paused a moment on his busy rounds to take a sip of his Pimm's Cup No. 1 -- a drink that was being offered by obsequious waiters circulating silently through the garden. He looked down into his glass, which contained a slice of lemon, a long strip of cucumber and a spring of mint besides the amber liquid. "It's just like drinking the vegetable plot," he said, and strode on to greet more of his guests. These did not include President Reagan, as had been hoped, but did include many of his associates, including his counselor, Edwin M. Meese III.

Invitations to the $125-a-plate (or, rather, $125-a-basket) picnic had specified "Victorian or Garden Party Dress," and many of the guests had chosen the Victorian option, perhaps as an index of where their hearts were. Richard Helms, for example, stood out in a shirt with blue and white vertical stripes, set off by a detachable white collar.

Lady Mary Henderson, the ambassador's wife, had a large, black ribbon with "HMS Pinafore" printed on it pinned around her straw hat, with roses at the back. Her dress, naturally, was a floor-length pinafore -- blue with a fleur-de-lis print and lace trim. "It's Laura Ashley's," she said, refering to a British store with a Georgetown branch that is ready to outfit hostesses for Victorian picnics. Instead of a purse, she carried a small woven basket, which contained her white gloves. (White gloves abounded at the picnic, but not as much as parasols and very large hats, often with flowers.) She wore an antique silver watch hanging from her waist on an antique silver chain.

A striking exception to the profusion of Liberty prints (from a London shop that carries an extensive line of Victorian-style fabrics) was the long, white dress worn by Mrs. Livingston Biddle and embroidered with a variety of flowers in a free design. "My sister embroidered it," she told an admirer. Also home-tailored, though less lavish, was the gown worn by Susan Miller, a medieval scholar and associate of the Folger Library who designs and makes her own clothes. "It cost me about $40," she said. "The cloth, from the G St. Remnant Store, cost almost nothing. The expense was mostly for the lace."

More than 420 picnic baskets were prepared, covered with napkins, ornamented with ribbons and containing (on a non-Victorian, plastic covered dish) a slice of pate, chicken tarragon, fresh grapes, two types of cheese and a roll. A few were left untouched, perhaps for a reason given by one of the guests who seemed more interested in the well-stocked bars."You can't eat much in these damned corsets," she said, "but you can drink."

Two other embassies (Australia and New Zealand) had been ready to help with an indoor picnic if the weather had been uncooperative yesterday, but they were not needed. "Our director of development, Jim Elder, was requesting prayers last Thursday," a Folger staff member said. "Nobody thought they would have much influence, but he said Mrs. Weinberger could do it."