"This black skeleton will make a white spectre."
A book of optical illusions published by J.H. Brown in 1864 was called “Spectropia or Surprising Spectral Illusions Showing Ghosts Everywhere and of any Colour”.
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Mr. Brown explained that if you stared at one of his images for 15 seconds and then moved your eyes to a white surface, you'd see an afterimage produced by the complimentary color of the original image. (And he was right.)
The book had a serious intention. Mr. Brown wanted to stop the "mental epidemic" of Spiritualism, the widely-held belief of his time that those who died could be contacted by spirit mediums. He wanted to debunk this belief by showing his readers how ghosts could be seen scientifically through optical illusions.
Found via a post at the blog Morbid Anatomy: Spectropia - Mirage and Ghost Stories at the Morbid Anatomy Library: Guest Post by Laetitia Barbier, Morbid Anatomy>>