Spectropia

'Spectropia' is an optical illusion book by J. H. Brown. He was distressed by an increased public interest in spiritualism, something which he deemed a "mental epidemic." By offering a scientific explanation on the properties of light, color, and the structure of the eyes, he states in the section 'Popular and Scientific Description' that the purpose in writing 'Spectropia' was to bring forth "the extinction of the superstitious belief that apparitions are actual spirits, by showing some of the ways our senses may be deceived."