William H. Mumler was a spirit photographer, someone whose camera could capture the ghosts of loved ones in images of the living person. Mumler took advantage of the spiritualism craze during and after the Civil War, as people wished to somehow connect with their loved ones who had died, especially during the war. Mumler came by this trade "honestly" as his wife was a healing spiritual medium.

Mumler's most famous photograph was one of Mary Todd Lincoln taken in 1869 that appears to show the ghost of Abraham standing behind her, with his hands on her shoulders.

There were skeptics. In 1869, Mumler was brought to trial as a fraud and huckster. Ironically, another huckster, Connecticut Legislator P.T. Barnum, testified against him and even produced his own version of Mumler's work, which was introduced in evidence against Mumler:

But prosecutors were unable to produce any conclusive evidence that the photographs were doctored and Mumler was acquitted.
Mumler continued to work with photography and even discovered a process, which was named after him, for producing and printing photo-electrotype plates as easily as woodcuts.