Stereoscope
2 of the 6 encyclopedias on this shelf carry an entry for Stereoscope. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
a simple and once the Caspian and Aral seas, between the popular optical contrivance, by which two slightly dissimilar pictures of an object are fused into one image, having the actual appearance of relief. A reflecting form of the stereoscope was invented by Professor Wheatstone in 1838. Subsequently Sir David Brewster invented the refracting or lenticular stereoscope, based on the refractive properties of semidouble convex lenses; and this instrument, of which there are numerous forms came into general use. Convex lenses magnify the pictures besides producing a stereoscopic effect. Photography greatly assists the stereoscope in providing perfectly accurate right-andleft monocular views, which are taken simultaneously sly on a plate in a twin camera. Several modifications of the re- Cyc-Vol. IX flecting stereoscope are distinguished by the names pseudoscope, iconoscope, telestereoscope, and polistereoscope, the last being an apparatus which serves the purposes of all the others.
Aiton's Encyclopedia (1910)
an optical instrument used in the examination of double photographic pictures made for the purpose. It was invented by Wheatstone in 1838. The original instrument was based on two mirrors, which have been replaced, however, by two refracting lenses. With the aid of this instrument two photographic pictures, a left hand and a right hand view, are examined at the same time, one by each eye, so that they make an impression of one picture. In this way a picture may be made to stand out more clearly. Surfaces appear in relief instead of flat, and relative distances are shown up better than by the examination of a single photograph, however clear it may be.
What Has Changed in the Last Century
The optics described here have not been revised: Wheatstone's mirror stereoscope of 1838 and Brewster's lenticular version remain the textbook account, and every modern 3-D display still works by feeding each eye its own slightly dissimilar view. What has vanished is the setting. When Collier's called it "a popular optical contrivance" in 1921 the hand-held card viewer was still standard parlour furniture; it faded through the 1920s and 30s, surviving mainly as the View-Master disc reel introduced in 1939 and as a working tool for reading overlapping aerial photographs in wartime and in map-making. The same principle returned as mass entertainment in the strapped-on virtual-reality headset, with the Oculus Rift and its successors from the 2010s doing electronically what Brewster's lenses did with two paper prints. Collier's list of variants "pseudoscope, iconoscope, telestereoscope, polistereoscope" is the one genuinely dated passage; those names have dropped out of use, and iconoscope was later reused for an early television camera tube.
Written for this edition, 2026.