Before revision: The directions normal to these planes—that is, the directions of a single wave speed for all polarizations—are called the binormal axesLandau & Lifshitz, 1960, p. 326. or optic axes,Born & Wolf, 2002, p. 801; Jenkins & White, 1976, pp.
After revision: twoThe directions normal to these planes—that is, the directions of a single wave speed for all polarizations—are called the binormal axesLandau & Lifshitz, 1960, p. 326. or optic axes,Born & Wolf, 2002, p. 801; Jenkins & White, 1976, pp.