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/ A device is described t hat permits t he rapid determination of the eq uivalent focallcngth of a lens. A t ran smitting biprism mounted betwcen a colli mated light source and a lens dividcs t he light incident upon t he front of thc lcns into two parallel beams m aking a fixed angle with one anoth cr. On passing t hrough the lcns, t wo imagcs arc formcd in t he focal plane. T he magni tude of t he lateral separation of t hc imagcs is dctermined by t he a nD" ular separation of t hc two in cident beams and the focallcngth of the lens. The fo cal length of t he imaging lens m ay be detcrmi ned from the measured separation of t he images at the focal p lane of t he, lens a nd the known angle of d eviation of t hc two incident beams produced by t he biprism.

Biprism method of determining the equivalent focal length of flat field lenses