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Optical device consisting of two prisms of very small refracting power, set base to base and which forms two images of a single source. It is often used to produce interference fringes. It may also be used to measure cyclophoria. Syn. double prism; Fresnel's double prism; Maddox bi-prism; Maddox double prism. See interference fringes; Wollaston prism; double prism test.