One of the stranger traditions in photography is "spirit photography", dating back (at least) to William Mumler's work in the 1860's. It was taken very seriously in its day; Sir Arthur Conan Doyle was a big believer in it, as was Alfred Russell Wallace, co-discoverer of the theory of evolution. Mary Todd Lincoln had her portrait taken with the "spirit" of her deceased husband Abraham behind her.

These days it's more an amusement. My students and I made quite a few pinhole spirit photographs when I was teaching high school photography, and it's still a fun thing to do with friends who drop by for wet plate portraits. Here are a couple from yesterday, keeping the 150+ year old tradition alive...