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conoscopic figureA conoscopic figure, or interference figure, is a pattern of birefringent colors crossed by dark bands (or isogyres), which can be produced using a petrographic microscope, or other source of crossed polarized light, for the purposes of mineral identification and investigation of mineral optical and chemical properties. With a microscope, this configuration is produced by interposing a Bertrand lens between the eyepiece and the objective, to view the rear (top) focal plane of the objective. This could also be accomplished using a phase telescope in place of the eyepiece. In older antique microscopes, conoscopic figures were produced using a crystal in front of an eyecap analyzer, thus no view of the objective was required. Production of such figures does not require a microscope though, and can also be accomplished simply by viewing a suitable crystal thin section between crossed polarizers with a light source. The phenomenon of conoscopic figures was known even in the early 19th century, being discovered by Sir David Brewster in 1813.