Thaumatrope

A thaumatrope is an optical toy introduced in 1825, consisting of a small disk with a different illustration on each of its two faces and two pieces of string attached to opposite edges; when the strings are twirled rapidly between the fingers, the disk spins fast enough that the two images, seen briefly one after another, appear to blend into a single combined picture, an effect that depends on visual persistence, the tendency of an image to linger in perception for roughly one-eighth of a second after it is no longer actually being seen. Common thaumatrope designs paired complementary images, such as a bare tree on one face and its leaves on the other, or a bird on one face and an empty cage on the other, and many examples combined their picture with a riddle or a short poem. The British physician John Ayrton Paris is usually credited with the invention, having described the toy in his 1827 children's science book Philosophy in Sport made Science in Earnest, though the mathematician Charles Babbage later recalled that the geologist William Henry Fitton was the toy's actual inventor, arrived at after a discussion with Babbage about a coin-spinning trick devised by John Herschel. The first commercial thaumatrope was published in London on April 2, 1825 by W. Phillips, sold in boxed sets of twelve discs at a price of seven shillings, and Paris reportedly earned about one hundred fifty pounds from sales in the United Kingdom before cheaper pirated copies of the toy flooded the market; historically, the thaumatrope is regarded as an important predecessor of motion pictures and animation, predating later animation devices such as the phenakistiscope. Its name derives from Ancient Greek and means wonder turner.