In this book from 1864, readers are asked to stare at the spectral images for unblinking, and then to dim the gas lamp or candle light and look at a white wall. Wondrously, the ghostly image appears to float on the wall right in front of the reader! This is an afterimage which has been “burned” on the retina. The author offers a scientific explanation by describing the structure of the eyes and the properties of light and color. The author ultimately attempts to debunk the belief in ghosts: “One thing we hope in come measure to further in the following pages, is the extinction of the superstitious belief that apparitions are actual spirits, by showing some of the many ways in which our senses may be deceived;” in this case: optical illusions.
If you are interested in seeing some specters, try this optical illusion for yourself by opening this image. Stare at the asterisk under the skeleton’s chin for 20 or more seconds. Dim the lights and stare at a white, blank wall or a white sheet of paper.