Samuel W. Doble was 16 when he walked into Shattuck’s photo studio in Lowell, Mass., in December of 1861.

His regiment, the 12th Maine, was en route to the front, and he was Company D’s drummer. He hadn’t been in the Army long; the 12th had just been formed. And the hardship of military life had yet to take its toll on him.

So as he stood before the camera, Samuel wore his cap tilted confidently to one side and his big drum slung from a harness around his neck. He adopted a look of manly determination.

But his face was that of a child, smooth and delicate, his cheeks and lips tinted pink by the photographer afterward.

Such were many of the lads who marched away in 1861. Hundreds of them, young and not so young, are returning in an exhibit at the Library of Congress that opens April 12 in the Jefferson Building.

Titled “The Last Full Measure: Civil War Photographs from the Liljenquist Family Collection,” the exhibit features 400 haunting pictures of the average Billy Yanks and Johnny Rebs who fought each other so bitterly from Bull Run to Appomattox.

There are images of men, or boys, with looks of serenity, of confidence, of innocence, as they stand before photographers to get the equivalent of a snapshot to send home to family or sweethearts, perhaps for the last time.

One photo depicts a handsome young Confederate soldier wearing a bow tie and with a watch fob. The inscription on the back says, “given to me by my Darling . . . Bobbie. Died Oct. 5th 1862.”

There are men in beards, youngsters in ill-fitting hats and guys goofing off. There are dandies and hicks, officers and privates, those who look like country boys and those who might be city kids.

They are captured in one-time-only moments from 150 years ago.

Library experts said that usually only a single image was made. It was framed in lacquered brass and placed in a special, velvet-lined case that looked like a small book, and the soldier walked out the door with the picture in his pocket.

They are “one of a kind,” said Dana Hemmenway, a conservator who examined all the photos on exhibit. “They didn’t make copies.

“It was really awe-inspiring to see them all,” she said. “Just peering into the faces . . . trying to imagine what their lives were like. Some of the faces looked very 19th century. Some of them looked extremely contemporary, like they would be somebody you’d pass walking down the street.”

Together, the photographs constitute a mosaic of the Civil War generation, which gave at least 620,000 lives in the creation of a new United States.

Here, too, are images of its children, wives, sisters and brothers, sometimes displayed with a lock of hair, a fragment of Shakespeare, a love poem, a piece of lace clutched in death on the battlefield.

Unlike Samuel Doble, whose name and particulars are scrawled on the back of his picture case and whose records the library has, most of the men in the images are unidentified.

The pictures are from the collection of the McLean jeweler Tom Liljenquist and his sons, who donated 700 glass ambrotypes and metal tintypes to the library last year. The family has been collecting the photographs for 15 years.

The exhibit is timed to coincide with the start of the national observance of the sesquicentennial of the Civil War, which began 150 years ago on April 12.

Most of the photographs are small, some not much bigger than a pack of matches. They are arranged in neat rows inside glass cases in a way that almost gives the effect of a quilt.

The library is also setting up two interactive stations at the exhibit where the pictures can be uploaded onto a computer screen and then enlarged to reveal the most minute details.

The same thing can be done online at the library’s Web site,

One day last week, library experts demonstrated the system using a picture of an unidentified little girl who had been photographed holding an image of what is believed to be her deceased soldier-father.

She is about 6. Her hair is parted down the middle and combed behind her ears, and she is wearing mourning ribbons on the sleeves of her dress. She sits with her hands clasped, and stares with a look of hurt on her face.

“Look at those eyes,” Susan K. Mordan, a library education specialist, said as she enlarged the child’s face on the computer screen. “You can almost see the studio photographer reflected in her eyes.”

Along with pain. There is an urge to speak to the girl, to say, “What is that, that you’re going through?” Mordan said.

Indeed, such interaction, in a way, returns the subject to life, Hemmenway said: With viewing, the faces “are brought back and made live again.”

For better, and worse.

The drummer boy, Samuel Doble, for example, who records show stood only 5 feet tall, did not take well to soldiering in the months after his picture was taken.

Much loved by the men of Company D and watched over by his father, the company cook, he suffered from chronic diarrhea, a deadly Civil War malaise that killed many a soldier. He was sent home in the summer.

But in 1863, two years older, four inches taller, and a “veteran,” according to the records, he reenlisted in a cavalry outfit. A few weeks later, he was severely injured when his horse fell on him. Although he could no longer fire his carbine because of the pain caused by its recoil, he remained in the Army until 1865.

Doble married, got divorced after the war, remarried, and raised two sons. Troubled by his injuries much of his life, he died in San Francisco, where he had been living with one of his children, on March 6, 1925, at the age of 79.

Now, 86 years later, he lives again — heading off to war, clutching his drumsticks, and eternally 16.