A thaumatrope is a toy, popular in Victorian times, that uses this persistence of vision. This illusion takes advantage of something called "persistence of vision". When an image is shown to your eyes, the retina keeps responding for a short time (about 1/30th of a second) after the image itself has gone away.

What is a thaumatrope and how did it work?

A thaumatrope is an optical toy that was popular in the 19th century. A disk with a picture on each side is attached to two pieces of string. When the strings are twirled quickly between the fingers the two pictures appear to blend into one due to the persistence of vision.

What was the purpose of the thaumatrope?

The thaumatrope was initially used to illustrate the phenomenon of retinal persistence, or persistence of vision: the idea that continuous motion was perceived because images remained on the retina for a period after the image was seen (Clee 71).