Thaumatrope

History:
The invention of the thaumatrope, whose name means "turning marvel" or "wonder turner," has often been credited to the astronomer Sir John Herschel.  However, it was a well-known London physicist, Dr. John A. Paris, who made this toy popular.  Thaumatropes were the first of many optical toys, simple devices that continued to provide animated entertainment until the development of modern cinema. 

How it works:
A thaumatrope is a small disc, held on opposite sides of its circumference by pieces of string.  An image is drawn on each side of the disc, and is selected in such a way that when the disc is spun, the two images appear to become superimposed.  To spin the disc, one string is held in a hand, and the disc is rotated to wind the string.  Then, both strings are held, and the disc is allowed to rotate. Gently stretching the strings will ensure that they continue to unwind and rewind.  This motion causes the disc to rotate, first in one direction and then in the opposite.  The faster the disc rotates, the greater the clarity of the illusion.
Although the thaumatrope does not produce animated scenes, it relies on the same persistence of vision principle that other optical toys use to create illusions of motion.  Persistence of vision is the eye's ability to retain an image for roughly 1/20 of a second after the object is gone. In this case, the eye continues to see the two images on either side of the thaumatrope shortly after each has disappeared.  As the thaumatrope spins, the series of quick flashes is interpreted as one continuous image.
One example of a thaumatrope has a tree with bare branches on one side, and on the other, its leaves.  When spun, the tree appears to be full of leaves.  Another example has a bird on one side, and a cage on the other.  When spun, the bird appears to be in its cage.  The bird-cage pair of images were used on the first thaumatrope, and is the most common one seen on thaumatropes today.


What became of it:
Most pairs of thaumatrope images were pictures that did not imply motion, such as running animals or dancing people.  A thaumatrope could only take two images and merge them, essentially creating one still image from two.  The phenakistoscope was a great improvement on the thaumatrope, creating one moving image from several stills, and became the first optical toy to create a true illusion of motion.

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