"optic axis"@en-us . . . "06010381-n"^^ . "0"^^ . . . . "optic axis"^^ . . . . . "0"^^ . . "in a doubly refracting crystal, the line in the direction of which no double refraction occurs; \"a crystal may have either one or two optic axes\""@en-us . "106010381"^^ .