Description
In 1861 Oliver Wendell Holmes created and deliberately did not patent a handheld, streamlined, much more economical viewer than had been available before. The stereoscope, which dates from the 1850s, consisted of two prismatic lenses and a wooden stand to hold the stereo card. This type of stereoscope remained in production for a century, and there are still companies making them in limited production currently.
Principles
A simple stereoscope is limited in the size of the image that may be used. 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. The stereoscope is essentially an instrument in which two photographs of the same object, taken from slightly different angles, are simultaneously presented, one to each eye. This recreates the way which in natural vision, each eye is seeing the object from a slightly different angle since they are separated by several inches, which is what gives humans natural depth perception. Each picture is focused by a separate lens, and by showing each eye a photograph taken several inches apart from each other and focused on the same point, it recreates the natural effect of seeing things in three dimensions.
A moving image extension of the stereoscope has a large vertically mounted drum containing a wheel upon which are mounted a series of stereographic cards which form a moving picture. A gate restrains the cards, and when sufficient force is available to bend the card, it slips past the gate and into view, obscuring the initial picture. These coin-enabled devices were found in arcades in the late 19th and early 20th centuries and were operated by the viewer using a hand crank. These devices can still be seen and operated in some museums specializing in arcade equipment.
The stereoscope offers several advantages:
Using positive curvature (magnifying) lenses, the image’s focus point is changed from its short distance (about 30 to 40 cm) to a virtual distance at infinity. This allows the eyes’ focus to be consistent with the parallel lines of sight, significantly reducing eye strain.
The card image is magnified, offering a wider field of view and the ability to examine the detail of the photograph.
The viewer provides a partition between the images, avoiding a potential distraction to the user.
A stereo transparency viewer is a type of stereoscope that offers similar advantages, e.g. the View-Master.
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 additional decay. This results in stereo artefacts when the images are viewed. These artefacts compete in the mind resulting in a distraction from the 3D effect, eye strain and headaches.
