Josephine Mickelson, widow of Sidney Mickelson of Mickelson's Fine Picture Framing and Gallery, was driving home last week when she spied a rainbow so spectacular that it made the evening news.

"I could see that it was coming from the direction of Olney, where Sidney is buried," she recalls. "It was as if he were reaching out to reassure me that I was doing the right thing."

"The right thing," until recently, had eluded her. "We never talked about what he wanted me to do with the business or the art," says his savvy, London-born wife of 49 years, who nursed her husband at home before he died of cancer in July at age 74. "He probably didn't want to depress me."

But after months of agonizing, she'd made a decision: She would close the nearly century-old family business and the building that houses it, which her father-in-law had bought in a sidewalk sale in the 1920s, and dispose of the hundreds of works of art she'd found stashed away there. Many of them, she believes, were acquired by founder Samuel Mickelson (known as Pops) early in the century and have languished since, unlabeled and anonymous. "This is an accumulation, a mishmash," she says.

No wonder. Founded a few blocks away as Mickelson's Bargain House on F Street (the sign outside said, "We buy and sell books, gramophones, musical instruments, tools, typewriters"), the business grew into a Washington landmark after a move to G Street NW, when a picture-framing business was added by elder son Maurice. By the end of World War II, when Sidney returned from serving in Europe with the Army combat engineers, Mickelson's had become the city's most prosperous frame shop, eventually counting presidents from Roosevelt and Kennedy to Clinton among its clients.

In 1962, fulfilling a longtime dream, Sidney Mickelson added an art gallery, where he showed an eclectic group of works ranging from prints by George Bellows and Maurits Escher to paintings and prints by Washington realists Frank Wright, Bill Woodward and Prentiss Taylor and English artist Norman Ackroyd.

Impervious to art world trends, he stuck to artists who appealed to him, most of them realists. He was also apparently--like his father--an inveterate collector. Even longtime staff members were stunned at the stacks and portfolios of work he had accumulated.

Now, in a bold stroke, Josie Mickelson has sent notices to loyal customers that the shop is closing and that this weekend, starting today, she'll be selling hundreds of the paintings, drawings, watercolors and prints that she and the staff found stashed in nooks and crannies of the two three-story buildings that now house the gallery and framing operation. She is also selling the buildings, which have become hot properties. She would not discuss the buyer or other details of the sale.

Meanwhile, the staff has been working frantically to get the works framed and hung before today's opening. "We're just trying to get everything on the walls," says gallery director Laura Graham.

Graham admits that the information the store can provide for many works ranges from scant to nonexistent. "There's been very little time to digest all this stuff," she says. "We don't have the staff to do research. Where did it all come from? I have no idea. It's all part of the inventory that most of us had never seen until now."

Anyone hoping to find the more valuable prints by Bellows, Escher, Carol Summers et al. will be disappointed. Nor will they uncover the works of such other artists as Taylor, William Preston, Wright, Woodward and Ackroyd. "I learned enough from Sidney to know that it wouldn't be fair to the artists we've worked so hard to promote to throw their work into a show like this," Josie Mickelson says.

Recognizable names are few, but William Hogarth, Charles Dana Gibson and Currier & Ives are among them, as is a World War I battlefield artist with the enticing moniker Muirhead Bone.

Perhaps the greatest treasures to turn up in this cleanup operation have been family documents that demonstrate the remarkable entrepreneurial skills of founder Samuel Mickelson, a Russian immigrant. One dealt with his purchase of Francis Scott Key's secretary and bookcase, which he bought at auction here and resold in 1935, during the heart of the Depression, for a then-astounding $14,000. Another was a patent he was awarded for a rig he devised, called a hair-cutting gauge, that was designed to help barbers execute a proper Buster Brown haircut. Josie Mickelson hopes to place these items and others in a time capsule to be buried somewhere on the site.

Meanwhile, Bill Parker, the chief framer at Mickelson's for more than 30 years, is acquiring the framing part of the business and is negotiating for a new space nearby. Mrs. Mickelson has given him the gallery's name as a gesture of good will. He says he will proudly take the old Mickelson's signs along with him.

"I wanted to close the place gracefully," says Josie Mickelson, "and agonized over which direction to go. So many people said it was a landmark. Hopefully, they'll now be going to Bill Parker.

"But it's the end of an era as far as the family is concerned. I wish I had grandchildren who were interested, but not many people are interested in retail now. It's a new world out there."

Mickelson's final sale will be held at the gallery, 707 G St. NW, beginning today from noon to 8 p.m. It will continue Friday and Saturday from noon to 6 p.m. and Sunday from 1 to 4 p.m.

CAPTION: Mickelson's in the 1930s, when founder Samuel sold Francis Scott Key's desk for a remarkable $14,000.

CAPTION: Mickelson Gallery staffers examine one of the hundreds of paintings, drawings, watercolors and prints being sold this weekend as the nearly-century-old business closes its doors.