Illusions

Illusions are usually distinguished, as having some basis in outward physical facts, from delusions, which are purely subjective hallucinations, with no foundation save perverted imagination, or otherwise disordered faculties. Optical illusions are exemplified by the appearances connected with mirage. See OPTICAL ILLUSIONS, APPARITIONS, DREAMS, HALLUCINATIONS, INSANITY; and Sully's Illusions (Inter. Sc. Series, 1881).