Early 3D Stereoscopes
Mirror stereoscope invented in 1833 by Sir Charles Wheatstone of England. Wheatstone was investigating how humans use binocular vision to perceive depth. It is very noteworthy that this invention predates photography, so that Wheatstone had to use hand-drawn pictures (below) for stereoscopic test images. Note the mechanical lead-screw for adjusting left-vs-right image separation, or parallax.
These are samples of drawings which Wheatstone used to represent left and right perspective views of objects in a 3-dimensional scene. Since his stereoscope invention predated photography, Wheatstone had to use draftsman ability to manually draw the scenes using classic vanishing-point perspective techniques.
Direct-viewing stereoscope invented in 1844 by David Brewster of Scotland. The method of viewing stereo-photographs in a side-by-side format is the predecessor of modern stereoscopes.
Stereoscopic photograph viewers invented in 1862 by Oliver Wendall Holmes in the US. Simple devices like these started the popular trend for viewing stereo-photographs of far-away places in people's own homes.
An improved stereoscope for viewing multiple stereo-photographs invented in 1938 by Hans Gruber in US. This is the better known as the View-Master.
(Illustrations reproduced from Foundations of the Stereoscopic Cinema by Lenny Lipton, and Amazing 3D by Hal Morgan and Dan Symmes.)