The first viewer for stereoscopic images was invented by Sir Charles Wheatstone in 1833, when photography was not invented yet (first daguerreotypes appeared in 1837). From 1859 the industrial production of stereoscopes began to develop on the standard created by O.W. Holmes, similar to the one presented here down.

Stereoscope Keystone, USA (1904). Until the first decades of the 20th century this kind of stereoscope was very common in most of the middle class families.


The Stereoscopic photos are obtained by means of a camera with double lens. The couple of photos is taken from a slightly different point of view which corresponds to the distance between the eyes. When you observe the two pictures through the lens of the stereoscope, your mind mix the two images so to have a 3d effect.

Brewster style stereoscope (XIX sec.)
Stereoscope Idealoscope, France (XX sec.).
French tablepot stereo viewer (1890) . Gentlemen would drop coins through the top slot and then turn the right hand knob to view a series of paper stereo cards. The box inside of it holds two dozen cards on a wire frame linked together in a continuous loop.
Tabletop stereo viewers like this one originally stood in the waiting room of Paris brothels and stereo cards inside of them depicted the boarders working in the house  (link)
.A nice graphoscope Souvenir de Paris. This kind of viewer let you see both pictures magnified by a powerful lens and stereoscopic photos through the smaller lens further down. 
When folded up this graphoscope is cm. 28x17.
In the last century Tru Vue and View Master were very popular stereoscopic toys. The Tru Vue shows stereo pictures on films, the View Master utilises its famous round cards.
Two further vintage stereoscopes (made in France): Stereoscope Lestrade and Stereoscope Bruguiere.