Ghosts in the Lens, Tricks in the Darkroom

From the story:

"Posing for a carte de visite, the vicomte, after Disdéri had snapped several dour shots of him in the de rigueur black frock coat and top hat, decided he would remove his clothes, all except socks and shoes, don what looks very much like a hot water bottle on his head but was in fact some sort of helmet, hold a shield and pretend to be a ghost. His friend (raised eyebrows, forefinger scratching forehead) acts as if the apparition startles him. (He doesn't look half startled enough.) Disdéri also sloshed around some chemical on the exposed negative of the naked vicomte to make the image look less corporeal."

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And HERE as well.