In the book, The Case for Spirit Photography, Arthur Conan Doyle explains that the photos seen below show ectoplasm, a word used to define spirit presence, forming above the people in two different pictures that were taken one right after the other. The "ectoplasmic veiling" begins as an "ectoplasmic bag" (1st photo), then is filled in with an image of a person in spirit (2nd photo).
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Ectoplasmic Veiling Picture
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Ectoplasmic Veiling - Ectoplasmic Bag Picture