Doyle’s book, The Case for Spirit Photography, actually came toward the tail end of the ghost photography craze. The most common versions of Mumler’s spirit photos featured a living person with a ghostly presence hovering behind or alongside them. Mumler was sued for fraud and, though he was acquitted, his spirit photography business dried up. He was also closely connected to the spiritualist movement, which used his photos as proof that there was a spirit world. He was a true believer in spirit photography, having commissioned a number of them:The defense of spirit photography wasn’t Arthur Conan Doyle’s only public attempt to prove that the supernatural was real.