conical refraction

GNU Webster's 1913

1. n. (Opt.) the refraction of a ray of light into an infinite number of rays, forming a hollow cone. This occurs when a ray of light is passed through crystals of some substances, under certain circumstances. Conical refraction is of two kinds; external conical refraction, in which the ray issues from the crystal in the form of cone, the vertex of which is at the point of emergence; and internal conical refraction, in which the ray is changed into the form of cone on entering the crystal, from which it issues in the form of hollow cylinder. This singular phenomenon was first discovered by Sir W. R. Hamilton by mathematical reasoning alone, unaided by experiment.