The 3D viewer shown above is a descendent of the Holmes stereoscope, circa. 1860, pictured at left, and works similarly: each of the user's eyes view the respective image of a side-by-side pair.
A fundamental limitation of the Holmes stereoscope is imposed by the human eyes... they are 2.5" apart and cannot diverge. This means that the viewed stereo pair cannot be very large.
A reflecting stereoscope overcomes this size limitation with mirrors, thus allowing a high resolution monitor to take the place of the paper stereo card.