The Fresnel equation governing the propagation of electromagnetic waves for the most general linear constitutive law is derived. The wave normals are found to lie, in general, on a fourth order surface. When the constitutive coefficients satisfy the so-called reciprocity or closure relation, one can define a duality operator on the space of the two-forms. We prove that the closure relation is a sufficient condition for the reduction of the fourth order surface to the familiar second order light cone structure. We finally study whether this condition is also necessary.Comment: 13 pages. Phys. Rev. D, to appea

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: In this report, a problem of classifying eight-line arrangements on the real projective plane is discussed . By using the combinatorial geometric method of finding transversals for all given segments , all possible eight-line arrangements have been generated from the three sets of 14 kinds of samples of seven-line arrangements, which are different each other . As a result, 135 kinds of eight-line arrangements have been observed.

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