I came across the object shown in the picture. Binocular vision, but of a single image. Each eye is redirected to the central are via prisms (definitely) and a semi-transparent mirror (presumably). The plane of focus is somewhere in the gap between the bakelite body and the piece(s) of ground glass. Sure, even without stereo vision, binocular viewing is more comfortable than squinting, but, is it worth the extra complexity? Has anyone seen such a device before?