The relation between phase normal and ray vector (unit vector in the direction of energy flux) and the expression for the ray velocity vector in an arbitrary weakly anisotropic medium is derived. The relation allows unique determination of the ray vector for a given phase normal and vice versa. In weakly anisotropic media, phase normal and ray vector differ by a first-order quantity. This means that in any expression containing a product of one of the mentioned vectors with a first-order quantity, both vectors are interchangable. The relation between phase normal and ray vector is linear. Therefore, the relation cannot describe, in principle, triplication of qS wavefront.
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