When Spiritualism became a widespread religion in the mid to late 19th century, séances were common and the belief in ghosts was accepted by many. After all, the Civil War had resulted in mass casualties and people wanted to believe more than ever that their loved ones could still be contacted in the afterlife. With the advent of photography, they could now allegedly be captured on film.
This magical invention could apparently see things the human eye could not. The first of these Spiritualist photographers was William Mumler, who claimed to have unintentionally photographed an image of his long-deceased cousin. For many years he created photos of ghosts standing near or behind family members. Of course, not everyone was a believer. P.T. Barnum was among those who cried humbug. Barnum took Mumler to court, though he was exonerated as no proof could be produced that he was a fraud.
The concept of spirit photography merged with another photography craze: stereoview cards. Now the ghosts could be seen in 3D. The effect is so rich and effective that the viewer can see right through the ghost. Many of these follow themes, such as ghosts flying over sleeping children or the fear of seeing an actual ghost. Others use the double exposure technique to create dream sequences.
See more at HartzmanStereoviews.com