stereoscoop

A stereoscope, also spelled stereoscoop, is a device for viewing two offset photographs or drawings that, when seen together, produce the perception of a three-dimensional scene. The two images are captured from slightly different viewpoints corresponding to the left and right eye, and the viewer combines them to create depth.

The device was developed in the 19th century. Sir Charles Wheatstone introduced the first practical stereoscope in 1838, using a pair of mirrors to present each image to a different eye. In the following decades Sir David Brewster refined the design for widespread use, popularizing a lens-based variant and a fixed viewing distance that made stereo cards comfortable to view.

How a stereoscope works: a stereo card typically carries two nearly identical images side by side or as a single composite image. In mirror-based designs, each image is reflected into the corresponding eye; in lens-based variants, lenses magnify and align the images so the brain fuses them into a single three-dimensional scene. Accurate alignment and an appropriate interpupillary distance are important for a comfortable view.

Variants and products: the Wheatstone and Brewster designs are the major historical types. The View-Master, introduced in the mid-20th century, popularized circular reels containing paired images viewed through a fixed eyepiece. Stereo cards were widely produced for entertainment, travel scenes, and education.

Impact and modern context: stereoscopes were highly popular in the 19th and early 20th centuries but declined with advances in film and digital imaging. The term remains associated with historical devices and with modern applications in stereoscopic imaging, virtual reality, and 3D visualization.