Final answer:
A thaumatrope is a 19th-century toy that uses the principle of persistence of vision to blend two pictures into one when its strings are twirled rapidly.
Step-by-step explanation:
The thaumatrope is a simple optical illusion toy used in the 19th century that demonstrates the principle of persistence of vision. It consists of a small, round disc with pictures on either side attached to two strings.
When the strings are twirled quickly between the fingers, the two pictures appear to blend into one due to the persistence of vision.
This phenomenon occurs because the human eye retains an image for a fraction of a second after its source has been removed from sight, an effect applied in various forms across animation and film technologies.