Holmes Stereoscope

Circa 1870
A Holmes stereoscope, the most popular form of 19th-century stereoscope

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 today.

Principles
A simple stereoscope may be used with limited image size. 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 an instrument in which two photographs of the same object, taken from slightly different angles, are simultaneously presented to each eye. This recreates how in natural vision, each eye sees the object from a slightly different angle since they are separated by several inches, which gives humans natural depth perception. A separate lens focuses each picture, 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, on which a series of stereographic cards form a moving picture are mounted. 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 specialising 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. The disadvantages of stereo cards, slides or any other hard copy or print are that the two images will likely receive differing wear, scratches and additional decay. This results in stereo artefacts when the images are viewed. These artefacts compete in the mind, distracting from the 3D effect, eye strain and headaches.