One explanation is the photography technology of the time. Long exposure times meant people had to remain very still, so photographers sometimes used hidden supports or posing stands that could create strange shapes in the finished image.
Double exposure, slight movement, clothing folds, and shadows could also produce forms that resemble a hand. Because our brains are quick to recognize familiar shapes, once someone points out an “extra hand,” the illusion can become difficult to ignore.
Without the original photographic plate or notes from the photographer, the exact cause may never be known. Still, ordinary photographic effects provide several believable explanations, making a supernatural answer unnecessary.
