The story of "ghost photography"

 The story of "ghost photography" is a fascinating one that emerged in the mid-1800s, when advances in communication technology collided with the rise of spiritualism.

Spiritualists believed in the possibility of communicating with the deceased through a medium, and this belief was fueled even more during the American Civil War, when many people sought solace in the idea of reconnecting with lost loved ones.

William Mumler, a Boston amateur photographer, took advantage of the phenomenon by claiming he could capture images of ghosts with his camera. He gained attention in Boston by selling "spirit portraits" with his wife Hannah, a professional photographer and Spiritualist medium.

Accusations of fraud were levelled against him and he was brought to trial but acquitted.
The trial had the opposite effect as one would expect and his business flourished, with people from all over the world coming to his studio to catch images of their loved ones.

The mystery of Mumler’s techniques endures, and his legacy as a curious figure in the history of photography remains fascinating to this day.

Source:
The (Mostly) True Story of Ghost Photography, by Robbie Mitchell for Historic Mysteries.