Stereoscopic, or 3D photography, works because it is able to recreate the illusion of depth. Human eyes are set about two-and-a-half inches apart, so each eye sees an image slightly differently. If one takes two separate photographs that same distance apart, with a suitable viewer it is possible to recreate that illusion of depth. It comes as a surprise to many to learn that the idea of stereoscopy actually preceded photography. Binocular drawings were made by Giovanni Battista della Porta (1538-1615), whilst about the same period Jacopo Chimenti da Empoli (1554-1640) produced drawings side by side which clearly indicated his understanding of binocular vision. Early workers in this field include Fenton, who took photographs in Russia, when he visited there in 1852, and Jules Duboscq, who made stereoscopes and stereoscopic daguerreotypes. Duboscq in turn caused Antoine Claudet to become interested in stereoscopy; indeed, it was Claudet who patented stereoscopes in 1853. The stereoscope took off in a big way when Queen Victoria and Prince Albert observed one at the exhibition at the Crystal Palace, and Brewster presented her with a stereoscope made by Duboscq. This signalled the beginning of a huge trade in stereoscopes and images; it is estimated that by the mid eighteen-fifties over a million homes owned one. One of the most successful salesmen of stereoscopic cards was George Nottage, later Lord Mayor of London, his catalogues listing over one hundred thousand views. The most common process for making stereoscopic cards was the Albumen one, daguerreotype images being very rare.