Even 150 years later, the eerie spirit photographs taken by Boston photographer William Mumler pack an emotional punch. A mourning mother is visited by the angelic silhouette of her departed daughter, the young girl resting her tiny hand on her mother’s lap. A mutton-chopped widower, his head hung in grief, is comforted by the glowing soul of his loving wife, her hands draped across his heavy shoulders.

It’s not hard to understand why 19th-century Americans enamored with the growing Spiritualism movement would have believed that these photographic apparitions were real, even as high-profile skeptics like P.T. Barnum decried spirit photography as a sham.

When spirit photography appeared in the 1860s, the United States was reeling from the Civil War, which claimed an astonishing 620,000 lives. Deep in mourning, Americans were drawn to anyone who offered even a fleeting connection to the souls of their dearly departed. Self-proclaimed mediums performed seances in which the living could speak with the dead, and photographers like Mumler granted the wishes of the bereaved to see their lost sons or brothers one last time.

Peter Manseau, curator of American religious history at the Smithsonian Institution’s National Museum of American History, says Mumler was surely a fraud, although he doesn’t know exactly how the photographer managed his trick. As he notes in his book, The Apparitionists: A Tale of Phantoms, Fraud, Photography and the Man Who Captured Lincoln’s Ghost, he also doesn’t discount the healing function that Spiritualism served.

“It was a genuine religious movement that meant a lot to people a time when the nation was going through mourning and loss like it had never had before,” says Manseau.

Mumler was, in Manseau’s words, a “kitchen tinkerer”—an amateur chemist and incurable entrepreneur who once peddled his own homemade elixir for curing dyspepsia. Trained as a silver engraver, Mumler decided to try his hand at photography, this wondrous new technology that produced portraits that people would pay a whole dollar to purchase.

While taking self-portraits for practice, one of Mumler’s prints came back with an unexplainable aberration. Although he was “quite alone in the room” when the shot was taken, there appeared to be a figure at his side, a girl who was “made of light.” Mumler showed the photo to a spiritualist friend who confirmed that the girl in the image was almost certainly a ghost.

Manseau says that Mumler had a knack for self-promotion and his otherworldly photo was written up in popular spiritualist newspapers like the Banner of Light and also the mainstream press. Bostoners began lining up at his small portrait studio to pay as much as $10 for their likeness with a lost loved one.

“Mumler sold himself as someone who could not explain what was happening or why he was chosen to take these pictures,” says Manseau. “He was as astonished as everyone else that suddenly his camera could take pictures of ghosts.”

A visitor to Mumler’s studio would be told that there’s no guarantee that a departed soul would appear. Mumler didn’t “command the spirits,” says Manseau, they “came and went as they pleased.” And if a photograph didn’t come out as the customer expected—the ghost of an old woman instead of lost brother, perhaps—Mumler would help the client search their memory for other spirits who might be eager to commune with the living.

Since photography was such a new invention in the mid-19th century, few people had other photos to compare with the faint, blurry images of the ghosts. Did Great Aunt Winifred wear her hair in a bun? Probably!

Mumler’s spirit photography attracted skeptics from the start. Manipulating images was a known part of the photographic artform and other photographers were openly experimenting with double exposures and superimposed negatives, all of which could create the effect of Mumler’s spirit photography.

One day, the veteran Boston photographer J.W. Black arrived at Mumler’s studio and demanded a demonstration. He sat for a portrait and carefully watched every step of Mumler’s process, including the alchemy of the dark room.

As Manseau describes it in his book, “Black watched as his own dark outline appeared on the glass, its form not unlike the photograph he’d had taken of himself seated with his newspaper. But then another shape began to emerge. ‘My God!’ Black said. ‘Is it possible?’”

The shape took the ghostly form of a man standing behind Black’s shoulder. Was it the great photographer’s father, who died when Black was 13? Black didn’t stick around to explain. He offered to pay for the print, and when Mumler politely refused, Black walked back to his studio, still clasping the photograph.

But over time, the evidence against Mumler started to mount. In one case, Mumler created a spirit photograph for woman who had recently lost her brother in the Civil War. When the brother miraculously returned home alive, things got awkward. But instead of accusing Mumler of creating a fraudulent photo, the faithful woman blamed it on an “evil spirit” trying to deceive her.

Another case was harder to dodge. A man visiting Mumler’s studio recognized a female ghost as his wife, who was not only alive but recently had her portrait taken by Mumler. Wasn’t it obvious that Mumler was reusing old negatives and playing them off as ghosts?

Since things were getting hot in Boston, Mumler tried relocating to New York in 1869, but he was quickly arrested and tried for fraud. The New York prosecutors called a parade of expert witnesses who offered at least nine ways that Mumler could have used photographic trickery to produce his ghostly images.

P.T. Barnum, a certified expert on “suckers,” commissioned a fake photograph of himself with the ghost of Abraham Lincoln to present as damning evidence in the trial.

But the jury was unconvinced. Sure, there were a million ways that Mumler could have faked the photos, but no one had caught him in the act or provided concrete evidence that he used any of those methods. The defense also cast doubt in the minds of the jury about the presumed limits of photographic technology.

“The defense argued that human ingenuity can do all these things that a generation ago would have seemed like sheer magic,” says Manseau. “How can we say that photography cannot do this, too?”

Mumler was acquitted and returned to Boston. He shied away from spirit photography and refocused his efforts on the chemistry of photo development. He eventually invented a technique called the “Mumler process” that allowed the first photographs to be printed on newsprint, transforming the practice of journalism.

But before Mumler hung up his hat for good as the world’s most famous spirit photographer, he welcomed none other than Mary Todd Lincoln into his Boston studio. It was 1870, five years after her husband’s assassination. Despite the accusations of fraud against Mumler and other spiritual mediums, Americans like the former First Lady, still deep in mourning, wanted to believe.

Mumler’s famous portrait of Mary Todd Lincoln shows the diminutive widow dressed all in black, her small hands clasped on her lap, while behind her stands the tall, slender, bearded apparition of her fallen husband.

“It was the last photo taken of her in her life,” says Manseau. “No one could dissuade her that it did not mean that Abraham Lincoln was still by her side.”