The invention of the Brewster Stereoscope by the Scottish scientist Sir David Brewster in 1849 provided a template for all later stereoscopes.

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Underwood & Underwood sold most of its stereoscopic library to Keystone.

This purchase gave Keystone an archive of nearly one million views and made it "the unchallenged leader in the stereoscopic educational business." In 1932, Keystone began promoting the value of stereographs as therapy in correcting visual problems and established the Stereophthalmic Department to manage that business area.

Singley retired as president of the company in 1937 at the age of 72.

About that time, movies began to cut into the stereograph business, but the company held on thanks largely to its ophthalmic customers.

In 1922, an interactive plasticon opened at the Rivoli Theater in In 1932, Edwin H Land patented a process for producing polarized filters that eventually led to the development of full color 3-D movies.

This was possible because the left/right separation could be achieved using the polarizing filters rather than the color channel.

Underwood & Underwood pioneered the sale of boxed stereograph sets to homes, businesses, and libraries. Lloyd Singley, who had been a salesman for Underwood & Underwood during his time as a student at Allegheny College in 1887-89, began to operate the Keystone View Company out of his house in Meadville, PA.

The subject matter largely consisted of travel scenes and comics.

was the first to recognize that depth perception is obtained when each eye simultaneously receives one of two dissimilar images of the same object.