Stereoscope

In current practice, the images are prepared so that the scene appears to be beyond this virtual window, through which objects are sometimes allowed to protrude, but this was not always the custom.

The earliest stereoscopes, "both with reflecting mirrors and with refracting prisms", were invented by Sir Charles Wheatstone and constructed for him by optician R. Murray in 1832.

[3][4][5] In this presentation he used a pair of mirrors at 45 degree angles to the user's eyes, each reflecting a picture located off to the side.

[7] Brewster's personal contribution was the suggestion to use lenses for uniting the dissimilar pictures in 1849; and accordingly the lenticular stereoscope (lens-based) may fairly be said to be his invention.

[8] This allowed a reduction in size, creating hand-held devices, which became known as Brewster Stereoscopes, much admired by Queen Victoria when they were demonstrated at the Great Exhibition of 1851.

The belt can usually hold 50 paper card or glass stereoviews, but there are also large floor standing models for 100 or 200 views.

[12] In the mid-20th century the View-Master stereoscope (patented 1939), with its rotating cardboard disks containing image pairs, was popular first for 'virtual tourism' and then as a toy.

A more complex stereoscope uses a pair of horizontal periscope-like devices, allowing the use of larger images that can present more detailed information in a wider field of view.

These coin-enabled devices were found in arcades in the late 19th and early 20th century and were operated by the viewer using a hand crank.

Disadvantages of stereo cards, slides or any other hard copy or print are that the two images are likely to receive differing wear, scratches and other decay.