Everybody knows what Abraham Lincoln looks like.

Or do they?Since a small portrait believed to depict a young Lincoln surfaced several years ago, historians and collectors have clashed over the image's authenticity.

The 1843 daguerreotype is absent the future president's recognizable features: the trademark stovepipe hat, his scraggily beard and the Great Emancipator's sharp profile - all immortalized on the U.S. penny.

But whoever the man is, he bears a striking resemblance to Lincoln - with a thin smile and sharp eyes.

Doubters argue the physical characteristics are all wrong, from the ears to a sharp nose that don't match known portraits of the 16th president. Defenders say scientific tests prove the daguerreotype - a mirror-image photograph produced on silver or copper - is one of the earliest pictures ever taken of the future president.

"We think we know what Lincoln looked like in the last 15 years of his life. But this could change all of that," said Rick Wester, director of photography at Christie's auction house. "In order for people to accept this as Lincoln, they have to confront this idea that we know what Lincoln looks like."

Some of the 20th century's most powerful tools have been employed in the evaluation: computer imaging and forensic science, along with hours spent poring over historical documents.

The disputed photograph will go on the auction block in October at Christie's in New York. It is expected to fetch at least $200,000, and possibly up to $1 million. The sale is advertised as "Portrait of a Gentleman, Believed to be Abraham Lincoln."

"It's awfully hard to figure out how one is going to say definitively that it is or it isn't Lincoln. In the end, it will be a marketplace decision," said Harold Holzer, the author of 12 books on Lincoln and the Civil War.

Lincoln collectors Robert and Joan Hoffman, of Pittsford, N.Y., bought the picture in 1992 for an undisclosed amount from an antiques dealer who got it from the Wadsworth family, descendants of John Milton Hay - assistant secretary to Lincoln.

Such a lineage is ammunition for those who contend the photo, believed to have captured Lincoln when he was a member of the Illinois House of Representatives, is genuine. The last known authenticated photograph of Lincoln also came from the Hay Wadsworth Estate in the late 1960s.

Leading the skeptics is Lloyd Ostendorf of Dayton, Ohio, the co-author of "Lincoln in Photographs" and a renowned expert. "Anyone who buys it will pay a lot of money for a fake," he said.

Ostendorf said the physical features of the man in the picture were vastly different than Lincoln, pointing specifically to the ears, nose, shoulders and eyes.

Look at the hands - actually, the veins in the hands, said Ralph Leonard, a medical professor at Wake Forest University. The vein patterns of the man in the portrait did not match a cast of Lincoln's hands made in May 1860, he said.

"Nobody's vein pattern is the same from person to person. I can prove anatomically that it's not Lincoln with the vein pattern," he said.