Encouraged by the developments in his personal and professional life, Riis began to excel as a journalist. By 1878, having already switched papers several times, he found himself covering the police beat for the New York Tribune. The job often took Riis to the squalid, high-crime neighborhoods where just a few years earlier he had almost starved to death. He accompanied police officers and health inspectors “on their patrols into the darkest corners of New York’s tenements. Riis not only developed his writing by reporting on these journeys, but he also sharpened his devotion to reform. Seeing the circumstances under which so many of New York’s tenement dwellers lived moved him deeply, and he was in turn able to move his readers, letting them travel vicariously through his articles.”

In 1887, Riis read about an invention by two German chemists who had found a way “to take pictures by flashlight.” With such a tool, he hoped to visually document the dank and demeaning tenements, which bred not only crime but such deadly diseases as tuberculosis and cholera. “With the flash he would finally be able to show New Yorkers the dark side of the city,” Buk-Swienty reports.

The flashlight could illuminate dark alleyways and penetrate filthy, poorly lit tenement garrets. It could expose basement dives and overcrowded five-cent lodging houses, show the trash heaps where tramps had dug caves to shield themselves from the elements. Riis could show his audience—the wealthier segments of society, policymakers and voters, whose actions could translate into social reform—what they had never seen before.

Riis never considered himself a talented photographer. Yet the power of the images he shot is evident even today. (In the recent Rediscovering Jacob Riis: Exposure Journalism and Photography in Turn-of-the-Century New York, Bonnie Yochelson and Daniel Czitrom argue that his photos continue to frame the discussion about urban poverty, more than a century after he took them.)

Certainly their impact was not lost on Riis’s contemporaries. When an editor from Scribner’s magazine attended a lecture and slide show by Riis, he promptly offered a contract for a package deal: an exposé illustrated by the photographs. The story ran in the December 1889 issue. It was followed by an expanded version in book form, How the Other Half Lives, which included forty-four searing images. (Buk-Swienty ups the ante, peppering his own text with fifty-five of the photographs.) The book sold briskly, providing Riis with the elusive financial security he had sought. It also led to considerable fame—although not in his native Denmark, where Buk-Swienty had never encountered his name. Indeed, it wasn’t until 1994 that the author began delving into Riis’s life, and that was only because he covered an exhibition of the muckraker’s photographs at the Museum of the City of New York.

It is impossible to gauge the precise impact of Riis’s exposé. Clearly, though, the book made an enormous impression on the city’s future police commissioner, Theodore Roosevelt. As Buk-Swienty writes:

Riis became his mentor and his guide to the slums, leading Roosevelt on nightly expeditions, showing him how ‘the other half,’ a staggering two-thirds of the city’s population, lived . . . . The horrid conditions, previously unfathomable to Roosevelt, would influence him throughout his political career, shaping his distinctive brand of Progressivism. Roosevelt, who was rapidly accruing political clout, became an unusually powerful ally for Riis. Together they pressured the city into tearing down the worst tenements; they brought about the closing of the lodging houses run by the corrupt and brutal police, the shelter of last resort for the poorest, often homeless men.

For Riis, meanwhile, it was business as usual. A year after achieving wealth and fame, he was still on the streets of New York City. During a visit to the municipal health department, he learned of a report about elevated nitrate levels in the drinking water, probably caused by leaking sewers. Riis speculated that the pollution could lead to a cholera outbreak. Lugging his camera and notebook, he traveled up and down the Croton River for a week, noting where small towns along the river were dumping raw sewage. When he published the photographs accompanied by his text, the New York City Council members eventually earmarked large sums to purchase the land along the river to prevent illegal dumping.

Throughout the next decade, Riis delivered one book-length exposé after another. The Children of the Poor, Out of Mulberry Street: Stories of Tenement Life in New York City, and A Ten Years’ War all struck chords with average citizens and policymakers alike. In 1901, he solidified his hold on the American consciousness with his wildly popular autobiography, The Making of an American. By the time of his death in 1914, Riis was something of a national icon, whom Teddy Roosevelt had called “the best American I ever knew.”

Steve Weinberg is the author of eight nonfiction books. His most recent book, Taking on the Trust: How Ida Tarbell Brought Down John D. Rockefeller and Standard Oil, has just been published in a paperback edition.