One of the early applications of photography was to reveal truths that were beyond the ability of the human eye to perceive, such as in the time-motion studies of E. J. Marcey and Eadweard Muybridge. At about the same time that photography was developing so was a new religious movement in America, Spiritualism. Spiritualists believe that the human mind is immortal, and that it is possible for the living to communicate with the dead; the new medium of photography presented them with a way to see what they believed was otherwise invisible, the spirits of the departed.
From the 1850s onwards various likenesses of dead people began to appear in photographs, but it was the American, William Mumler, who in 1869 transformed spirit photography into a lucrative business. He offered to photograph his clients in the company of one or more ghosts, unseen when the exposure was made but visible as translucent images on the developed photograph.
The general public was perhaps more susceptible to the idea of communicating with the dead in the aftermath of the American Civil War, which had ended in 1865 after claiming up to 750,000 lives. A similar sense of loss following the Second World War in Europe led to the commercial success of English spirit photographers such as William Hope.