Reflecting stereoscope, 19th century.

UNITED KINGDOM - SEPTEMBER 04: Reflecting stereoscope, made by Sir Charles Wheatstone (1802-1875). Wheatstone demonstrated his stereoscope to the Royal Society in 1838 in order to create an apparently three-dimensional image to demonstrate binocular vision. Although Wheatstone�s invention was intended to be an experimental demonstration apparatus, stereoscopes became popular scientific toys. In the early 1840s, after the invention of photography, some of the foremost early photographers such as W H Fox Talbot and Roger Fenton began producing calotypes specifically for use in stereoscopes. The popularity of stereoscopes was such that 250,000 of the devices were sold in Paris and London over a three month period in 1851. (Photo by SSPL/Getty Images)