The photographing of ghosts or other spirits, popularized during the spiritualism era of the middle and late 19th century. In 1862, a Boston photographer, William H. Mumler, discovered on one of his photographs a human image besides that of his sitter. Although his subject, a Dr. Gardner, had been posing alone, the photographic plate showed an additional figure that Gardner identified as a long-deceased cousin. Gardner’s astonishment and subsequent publicizing of his experience led to more ghost photos by Mumler, some of which were clearly shown to be frauds perpetrated by double exposure of the photographic plate. Some, however, were not so easily explained.
During the next several decades, spirit photography increased. Extant photos of the era show amazing images of ghosted figures, both card board like and more realistic, and of spiritualists oozing ectoplasm from every visible orifice. Many of these were later shown to be the result of double exposures using portraits of deceased people and even using images of still-living persons, images projected onto the plates through the use of mechanical projectors or mirrors, or other tricks sometimes involving the collaboration of the sitter and sometimes not.