Binoviewer
A binoviewer is an optical device designed to enable binocular viewing through a single objective.
1 - Eyepiece 2 - Compensation slide
3 - Prism 4 - Beam splitter
5 - Body 6 - Barlow lens
In contrast to binoculars, it does not allow stereoscopic viewing, as both images are produced by the same objective and do not differ except for aberrations induced by the binoviewer itself. However, binocular viewing allows the brain's visual system to make use of binocular summation, resulting in a more relaxed viewing experience and an increased ability of detecting faint details.
A binoviewer consists of a beam splitter which splits the image provided by the objective into two identical (but fainter) copies, and a system of prisms or mirrors that relay the images to a pair of identical eyepieces.
Binoviewers are a standard component of laboratory microscopes and are also used with optical telescopes, particularly in amateur astronomy.