Spectropia was a book published in 1864 by JH Brown to demonstrate how the brain tricks people into believing they are seeing a ghost. The book contains 16 illustrations of “ghosts” and, if a reader stares at one illustration under bright light for 20 seconds and then looks at a blank wall in a dark room, a reverse-colored version of the image appears.
This optical illusion, known as an afterimage, is caused by the temporary fatigue of color-sensing cells in the retina. Brown argued that many alleged apparitions are produced in this way, aiming to debunk spiritualism, a religious movement popular in the 19th century suggesting the living could communicate with the dead.
Spectropia became popular as a tool against superstition and as a “philosophical toy”, raising questions about the nature of belief and perception.
