If you look closely at these photos -- and even if you don't -- there's more than a ghost of a chance that you might see what looks like spirits or apparitions.
The paranormal pics are the work of William H. Mumler, a mid-1800s engraver in Boston who rose to fame as a "spirit photographer."
Mumler's stock-in-trade was taking photos of people whose relatives died during the Civil War and making it look as if the spirits of their souls were also posing.
Mumler's most famous photo was of Mary Todd Lincoln with the ghostly image of President Abe Lincoln in the background. His work was so popular for a time that he was able to charge $10 for a picture at a time when other photographers charged a fraction of that, according to the American Philosophical Society.
Of course, those photographers didn't have ghosts in their shots.
But neither did Mumler, whose ghosts appeared via a double exposure technique he discovered while experimenting with a camera.
Mumler's spirit photography was a popular fad, but it also attracted critics like showman P.T. Barnum, who felt that Mumler and other spirit photographers were taking advantage of grief-stricken people.
In 1869, some people discovered that the so-called ghosts in Mumler's photos were actually living Bostonians, and Mumler was tried for fraud, according to MuseumOfHoaxes.com.
Barnum -- who had earlier purchased photos from Mumler to display -- testified against him and showed a photo done by a different photographer in order to demonstrate how easy it was to fake ghost photos.
Although the judge dropped the charges against Mumler, citing a lack of evidence, he did concede it was possible that the photographer had practiced "trick and deception."
Despite the apparent victory, Mumler spent $3,000 defending himself (the equivalent of $51,724) and was stuck in a financial hole he never got out of by the time he died in 1884, according to PhotographyMuseum.com.
Mumler's techniques for taking photos are primitive compared to the advances made in digital technology.
However, researcher Ben Radford says there is one thing his photos have in common with modern-day ghost pictures.
"Despite ever-increasing technology, real photographic proof of ghosts remains as elusive as ever," he wrote in LiveScience.com
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