Viewers are optic devices guiding each of both images of a stereo pair through the corresponding eyes. Three kind of optic elements could be used or combined.
Mirror (Wheastone stereoscope): The first ever stereoscope was patented by Charles Wheatstone in 1838, nearly at the same time a fantastic technique and form of expression was created ; nothing else than photography itself. Bad thing: In the original version, the two images are left-right mirrored. A more modern design adding a second pair of mirrors put the image the correct way.
Lenses (Brewster Stereoscope): In 1849, David Brewster (also inventor of the Kaleidoscope) invented the principle of the two lens stereoscope. Defect: Apparatuses generally quite bulky witch can be used just by one person at a time. Advantages: Good vision of 3D and ease of manufacturing. It provided a lot of mythical stereoscopes such as Vérascope, Bruguière and Lestrade Stereoscope, the ViewMaster and more recently 3D postcards (simple stereo-pair with two lenses in a folding cardboard box).
Prisms (Holmes Stereoscope): Starting from Brewster's work, Oliver Wendell Holmes set-up the stereoscope using prismatic lenses. It gave a device easy to carry, that was presented in a publication in 1859. He decided not to patent its creation, so he is a precursor of the authors of free softwares used by the majority of stereoscopists to mount images <;o).
Two small screens visor (Head Mounted Displays): This is the modern version of the viewers described before. In that case, it carry two small screens connected to a computer in front of each eyes. That system is very similar to the Head Mounted Displays included in the helmets of the fighter pilots. Advantage: It is possible to see moving pictures and also movies with that system. Defect: the spectator's head is set into a kind of box quite impressive.
This type of viewing the third dimension using viewers has the great defect to be individual. Of course, it could be considered that the kind of initiation, such as peace pipe, is an advantage: The viewer goes from hand to hand sharing slowly the three dimensions. But, when the group is more than half-dozen persons, the session is majority of waiting times. The remedy is to use projection-based methods.
