Note 13 — Chapter I · Introductory Optics for Optical Mineralogy.

Instead of ω and ε, for convenience in tables, etc., α and γ are used, denoting the indices of refraction of the rays traversing the crystal with greatest and least velocity respectively, without regard as to which is the O or E ray. A good reason for this convention is that the symbol (γ − α) is used to express in decimals the relative strength of the double refraction of a crystal, whether uniaxial or biaxial. γ is always greater than α.