Sometimes old photographs hold their secrets for years.

A noted Civil War picture called "Confederate Dead on Matthews Hill" at Bull Run showed four dead soldiers in a forest clearing. But about 25 years ago a historian found another shot of the same four men in the same clearing. And they were alive. The "corpses," apparently, were staged.

 Then there’s the well-known picture of the crowd at Gettysburg for the dedication of the Civil War cemetery. A few decades ago someone examined it with a magnifying glass and discovered that it actually showed Abraham Lincoln, a tiny bareheaded figure, probably looking over the speech itself. This is the only image of him at Gettysburg that day.

Sometimes finding the old photographs themselves is an adventure. Laura Kreiss, a picture researcher for "The American Presidency: A Glorious Burden," the new permanent exhibition at the National Museum of American History, recalled her ordeal in trying to locate a picture of Presidential nominee William Jennings Bryan speaking to a group of loggers from a stump in 1896—literally, a stump speech.

"For days I turned the place upside down," Kreiss said. "It wasn’t under ‘Presidential Elections,’ wasn’t under ‘Bryan,’ or ‘Campaigns.’ I mentioned this to a staff member. It turns out it was in a file cabinet that had been moved to a temporary storage area." The photograph is now featured in the media section of the exhibition.

Finding the right pictures can be a lifework of frustration and triumph. The problems are obvious: Exactly how do you categorize a particular photograph and where do you file it? Many institutions and agencies don’t have universal cataloging rules; how a picture is classified seems to depend mostly on the person filing it.

In Washington the search for photographs is a thriving business, for the city is up to here in old pictures.

"The Smithsonian alone has about six million photographs in various archives," said Kreiss, who worked on a massive inventory of the Institution’s objects in the 1980s.

But a photo archive is a bit like the Egyptian desert—no one knows what ancient wonders it holds. It takes an expert to unearth these treasures. "You have to rely on good archivists who know their collections," said Kreiss, who has been freelancing for the Smithsonian for 20 years.

"You want, say, some farmers picking cotton in the 1940s. Which repository would have them? Well, probably several in Washington. I would check the Department of Agriculture, the Library of Congress, the National Archives and the Museum of American History. It can be very labor intensive, especially if you’re trying to find the best photographs."

 For "The American Presidency," Kreiss and colleagues Carrie Bruns, Shannon Perich and Sue Ostroff dug up some 400 photographs, engravings and lithographs to accompany a plethora of Presidential artifacts, such as the lap desk on which Thomas Jefferson penned the Declaration of Independence, Lincoln’s top hat, a bullet-pierced eyeglass case that saved Theodore Roosevelt’s life when it deflected an assassin’s bullet, a filing cabinet—its drawers crudely crowbarred open—from Richard Nixon’s scandals, and even Warren Harding’s monogrammed silk pajamas.

At the behest of Smithsonian Secretary Lawrence Small, the exhibition was put together in eight months; normally it would take at least three years. Fortunately, most of the 900 objects in the show were in the Smithsonian’s legendary "attic." And about 80 percent of the photographs are from Smithsonian repositories.

A couple of hours poring over old photographs sounds like fun to me. Kreiss says you come across some great things you weren’t looking for.

While she was scouring the National Archives on a project for the Women’s Museum in Dallas, she turned up some 1920s photographs predicting what women would be wearing in 2000: "soft metal" dresses, frocks with built-in wings for flying, and electric coats. Kreiss took me on a tour of "The American Presidency" to show me some of the photographic gems that bring the exhibition together. What fascinated me was the imaginative use made of some pictures. A shot of Harry Truman on a whistle-stop train tour was blown up to life-size and curved to look just like the real thing.

The original print of James Buchanan taking the oath of office outside the Capitol in 1857, the oldest known photograph of a Presidential inauguration, is here in a glass case. The print is very fragile and will be removed from the display and replaced by a durable modern copy.

I was equally taken with a grand indoor scene: an image of the old Pension Building, now the National Building Museum, all rigged out for William McKinley’s inaugural ball in 1901. Equipped with electricity for the first time, the place is ablaze with strings and spirals of brilliant lights, making the enormous columns shine.

An alcove of the exhibition devoted to life in the White House is sure to please youngsters with its display of such personal items as an 1829 bisque doll belonging to John Quincy Adams’ granddaughter, the elaborate dollhouses of Jimmy Carter’s daughter, Amy, and Grover Cleveland’s children, a découpaged paint box, pointe shoes and a chess set. The photographs accompanying this section enhance the domestic setting: there are pictures of Commanders in Chief cuddling their grandchildren, of Lincoln reading to his son Tad, and of children having a snowball fight on the White House lawn. And, of course, there’s that charming picture of John Kennedy, Jr., playing under his father’s desk.

"Many of these pictures are from Presidential libraries," Kreiss said. "The hardest to get were of Chelsea. The Clintons really protected her."

But apparently some Presidents are less concerned about their privacy: here also is a picture of Lyndon Johnson conducting a staff meeting from his bed, a habit of his, Kreiss said.

 Occasionally, certain photographs prove to be elusive, even though everyone thinks they’ve seen them everywhere. "We needed a shot of Franklin Roosevelt with his dog Fala," Kreiss said. "Easy, we thought. But the National Archives didn’t have one. We called the wire services. Nope. The Roosevelt Presidential Library had a picture but couldn’t give us permission to use it. We finally found a copy at one of the big stock houses that lend commercially."

The long-sought-after photograph shows the Scottish terrier in the front seat of a convertible with FDR at the wheel (the President, disabled, used hand controls to drive).

One of the most compelling pictures is an enlargement of Ulysses S. Grant in his post-Presidency days. Suffering from throat cancer, he is shown swaddled in blankets, working feverishly on his memoirs, which he wanted to finish before his death, in order to support his wife.

Laura Kreiss has been finding photographs most of her life. "I have a master’s degree in anthropology," she told me, "but I’ve been interested in photographs since I was a little girl. I got my first job working for the National Institutes of Health with a medical researcher who set up a film and a photographic archive at NIH."

A few years later, when Kreiss was working at the National Library of Medicine at NIH, Woody Allen’s people called and wanted footage of mental institutions in the 1930s. "They were researching the film Zelig. I helped them find some film footage and got a credit in the movie. Boy, I thought, finding this stuff is so much more interesting than just storing films and putting numbers on things."

These days, Kreiss sticks to still pictures, usually from the Smithsonian or the Library of Congress, which she knows very well. No, she hasn’t worked for Ken Burns, the movie documentarian, but she has done research for public television shows and museums across the country.

"A lot of things are on the Internet now," Kreiss said, "and there are several on-line catalogs, but usually they’re just the tip of the iceberg of what each repository has." To find the best shots, she usually follows up with an actual visit to the repository. "I love historical photographs. They’re a window to the past," she said. "I have this unusual view of history: it’s mostly visual. I know the image, but I’m not always sure of all the historical facts."

By Michael Kernan