Prismatic binoculars, c 1951.
© Science Museum / Science & Society Picture Library
These binoculars, with a half cut-away section, were exhibited at the Festival of Britain, London in 1951. Made by the optical company of Ros, London, they are typical of the prismatic binoculars being made at the time. Unlike opera glases that employ concave eyepiece lenses to produce upright images, these binoculars use porro prisms. Through the use of two 45-degree prisms in each tube, the image is made erect and the length of instrument is considerably shortened. Binoculars of this type are both compact and offer higher magnifications that are normally found in simple opera glases.