A stereogram is an optical illusion; a flat two dimensional image that reveals a three dimensional image if viewed in a certain way.
Some other forms of stereograms include Anaglyphs and Autostereograms.
In 1838 a British scientist and inventor, Charles Wheatstone, discovered the stereogram through his explanation of, binocular vision.
Binocular vision is the ability to maintain visual focus on an object with both eyes, thus creating a single visual image.
With a combination of prisms and mirrors, Wheatstone, designed and constructed a stereoscope. This is a viewing device which allows users to create a three-dimensional image from a set of two-dimensional photographs or drawings.
Later, in 1861, Oliver Wendell Holmes Sr, ( author, lecturer,physician and professor ), invented an improved form of stereoscope that became quite popular.
This new improved stereoscope was a simple device with a special viewer. It had a clip for holding special stereoscopic cards in place.
A person would peer through a binocular system, which forced each eye to see only one of the two images. By either crossing or diverging one's eyes, a third image would eventually appear in the middle.
They were a popular form of entertainment for people in the late 1800s and early 1900s.
Of course, with the emergence of computers in later years things changed a little.
Now computers were used to create autostereograms.
An autostereogram is a single-image stereogram (SIS), designed to create the visual illusion of a three-dimensional (3D) scene from a two-dimensional image in the human brain.
Just stare at the picture and find the image ^_^
