being stereoscopic as well. Based on the principles of stereopsis, described by Sir Charles Wheatstone in the 1830s, stereoscopic technology provides a different image to the viewer's left and right eyes. The following are some of the technical details and methodologies employed in some of the more notable stereoscopic systems that have been developed. Side-by-side images Traditional stereoscopic photography consists of creating a 3D illusion starting from a pair of 2D images, a stereogram. The easiest way to enhance depth perception in the brain is to provide the eyes of the viewer with two different images, representing two perspectives of the same object, with a minor deviation exactly equal to the perspectives that both eyes naturally receive in binocular vision. If eyestrain and distortion are to be avoided, each of the two 2D images preferably should be presented to each eye of the viewer so that any object at infinite distance seen by the viewer should be perceived by that eye while it is oriented straight ahead, the viewer's eyes being neither crossed nor diverging. When the picture contains no object at infinite distance, such as a horizon or a cloud, the pictures should be spaced correspondingly closer together. The side-by-side method is extremely simple to create, but it can be difficult or uncomfortable to view without optical aids. Stereoscope and stereographic cards A stereoscope is a device for viewing stereographic cards, which are cards that contain two separate images that are printed side by side to create the illusion of a three-dimensional image. Transparency viewers Pairs of stereo views printed on a transparent base are viewed by transmitted light. One advantage of transparency viewing is the opportunity for a wider, more realistic dynamic range than is practical with prints on an opaque base; another is that a wider field of view may be presented since the images, being illuminated from the rear, may be placed much closer to the lenses. The practice of viewing film-based stereoscopic transparencies dates to at least as early as 1931, when Tru-Vue began to market sets of stereo views on strips of 35 mm film that were fed through a hand-held Bakelite viewer. In 1939, a modified and miniaturized variation of this technology, employing cardboard disks containing seven pairs of small Kodachrome color film transparencies, was introduced as the View-Master. Head-mounted displays The user typically wears a helmet or glasses with two small LCD