Toronto. … and things that go bump in the night. An old saying to scare children silly in the days before electrical lighting. In the 19th century, few people understood exactly how photography worked. These folk often fell victim to those claiming to photograph spirits or ghosts.
In time, people discovered such things were simply double exposures either on the positive (print) or negative. In camera was easy especially when glass plates and cut film was used (or early roll film cameras). 20th century roll film cameras touted double exposure prevention as a perk.
By under exposing the “ghost” it would appear to be more realistic. The above image was taken with a Leica which had double exposure prevention provided the photographer did not reuse a previously exposed roll of film (my bad)! In later times I tore a notch in the film or wrote on the leader to avoid such unintended results.