A form of optical illusion originally introduced in 1838 by Charles Wheatstone, the stereograph derives from the fact that human beings see the world through two eyes, each of which sees a slightly different view. When the brain receives and combines these two images, the result allows us to perceive the world in three dimensions. In the late 1850s, photographers created special cameras with two lenses that reproduced the vision of two separate eyes. These cameras produced two negatives, side by side, on a single piece of glass. After the negatives were printed, and the resulting photographs mounted on special cards, these cards could be placed in a viewer, where they reproduced a startlingly lifelike image in three dimensions.
Stereographs remained a popular form of home entertainment from the 1860s through the 1920s. They can be seen as the precursor to newsreels, movies, and eventually television.
Admiral John Dahlgren/Mathew Brady Studio/Albumen silver print