In the late 1950s, a Swiss-born photographer named Robert Frank traveled America, documenting its days and nights. With a Guggenheim fellowship, Frank captured Americans in 48 states, on 767 rolls of film and almost 27,000 frames.

His images were published in 1958 in "The Americans," which many call the most important book of photography of the 20th century. Through April 26, all of the photographs in "The Americans" will be on display at the National Gallery of Art in Washington, D.C.

[View images from 'The Americans' and listen to curator Sarah Greenough talk about Frank and his work in this slide show.]

Frank was born in 1924 to a Jewish family and, through his father, with German citizenship. But as Hitler's ideology swept across Europe, the family's citizenship was stripped. Later in life, Frank admitted he often felt like an outsider looking in. Sarah Greenough, curator of photography at the National Gallery, believes this sense contributed to his feeling "separated and removed from society," but also enabled him to relate to the ostracized.

Travelling just before the 1960s, Frank witnessed a country on the precipice: Race relations were on the brink and a counterculture on the rise. He was here, Greenough said, to capture "the kind of civilization born here and spreading elsewhere."

From this civilization, Frank "sucked a sad poem right out of America onto film, taking rank among the tragic poets of the world," Jack Kerouac wrote in the introduction to "The Americans."

Frank spent months organizing the photos and slipping a little poetic justice into the sequences: America's plump rich abut its rural poor, glowing jukeboxes follow forlorn funeral-goers. As Kerouac wrote, "[Y]ou end up finally not knowing any more whether a jukebox is sadder than a coffin."

Curator Greenough agrees: "Does it show loneliness and isolation? Absolutely. I think Frank very much felt that as he was driving around the country. Would he say that's all there is to the American experience? Absolutely not."

Said Frank in 1951: ''When people look at my pictures I want them to feel the way they do when they want to read a line of a poem twice."

"Looking In: Robert Frank's 'The Americans'" will be at the National Gallery until April 26. Later it travels to the San Francisco Museum of Modern Art, May 16 to Aug. 23; and the "Metropolitan Museum of Art": http://www.metmuseum.org/ in New York, Sept. 22 to Dec. 27.