nα ; nβ ; nγ

n_α ; nβ ; nγ: in crystallographic optics, symbols indicating, in a Biaxial Birefringent stone (see also Optical Sign), respectively, the Refractive Index of the numerically lowest-valued Variable Ray (n_α), the Refractive Index of the Intermediate Ray (nβ) and the Refractive Index of the numerically highest-valued Variable Ray (nγ).

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