A book on optical illusions and “ghosts” from 1865 that includes a number of “plates” (images) of various spectral figures.
Stare at a fixed point on each plate for 15-20 seconds then stare at a blank surface (a light wall or ceiling) and “see” the ghostly figures before your eyes! (Blinking as you stare at the blank surface increases the effect.)
Spectropia ; or, Surprising spectral illusions. Showing ghosts everywhere, and of any colour