Brewster stereoscope
A Brewster stereoscope is a vision therapy instrument that uses a septum to give each eye its own half of a stereogram card. Its lenses and prisms let the clinician test and train binocular vision at different simulated distances.1
The usual optical system is a pair of +5.00 D spheres with optical centers about 95 mm apart. That separation is wider than the average interpupillary distance, so the lenses induce base-out prism.2 The card can sit anywhere from the distance setting, 20 cm with 5 D lenses, to any near point setting, and the therapist can therefore vary both the accommodative demand and the convergence demand.2
The Wheatstone stereoscope separates the two eyes' views with viewing tubes or mirrors instead of a septum.1 Wheatstone invented his stereoscope in 1838 and was the first to recognize that stereopsis arises when horizontally disparate retinal elements are stimulated simultaneously.3
More detail
The lens power and the card distance are both known, so the accommodative demand is easy to calculate. The demand equals the reciprocal of the card distance in meters minus the power of the stereoscope lenses in diopters.2 With +5.00 D lenses and the card at 20 cm, the demand is zero, which is why 20 cm is the distance setting for that instrument.2
Sources (7)
- Scheiman and Wick, Clinical Management of Binocular Vision, ch. 5
- Scheiman and Wick, Clinical Management of Binocular Vision, ch. 6
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 2
- Brewster, D. (1856) The stereoscope: its history, theory and construction, with its application to the fine and useful arts and to education. London: John Murray.
- Kemp, M. (1990) The science of art: optical themes in western art from Brunelleschi to Seurat. New Haven: Yale University Press.
- Stafford, B.M. and Terpak, F. (2001) Devices of wonder: from the world in a box to images on a screen. Los Angeles: Getty Research Institute.
- Yale Center for British Art (2023) Brewster stereoscope. Available at: https://collections.britishart.yale.edu/catalog/orbis:10174265 (Accessed: 5 May 2024). secondary
Reviewed October 2026