Corrections for mean indices of refraction in Scheme (insert folder): Scapolite 1.551 to 1.584; Calcite 1.601 and transfer to Dolomite rectangle; Dolomite 1.622; Corundum 1.766; Tourmaline 1.633 to 1.674. ----- Footnote 1: Often called a direction of maximum elasticity, c′ being a direction of minimum elasticity. Footnote 2: Rosenbusch’s _Microskopische Physiographie_, p. 156. Footnote 3: For a more complete discussion of optics, in connection with Optical Mineralogy, the student is referred to A. J. Moses’ _Characters of Crystals_, p. 85, et seq.; Moses’ & Parsons’ _Mineralogy, Crystallography and Blowpipe Analysis_, Chap. XVI., 4th Ed., 1909; Miers’ _Mineralogy_, 1902; L. Fletcher’s _Optical Indicatrix_, etc., 1892; Groth’s _Physikalische Krystallographie_, 3d Ed.; Rosenbusch’s _Mikroskopische Physiographie_, 4th Ed. and Iddings’ _Rock Minerals_, 1911. Footnote 4: The electromagnetic theory of light is now very generally held, but, whatever may be the recurrent change of state to which light is really due, the principles of wave motion furnish a satisfactory geometric description of optical phenomena. Footnote 5: These sections are supposed to have plane parallel faces, such being the case in ordinary practice, and to be examined with parallel perpendicularly incident light. Footnote 6: It is interesting to remember in this connection that in the isometric system there is also the greatest possible symmetry of “form.” Footnote 7: A. J. Moses, _Characters of Crystals_, pp. 85–97. Footnote 8: For this branch of optical physics, see A. J. Moses, _Characters of Crystals_, pp. 97–100. Footnote 9: This can be demonstrated by using a nicol and a plate of calcite which shows a double image. If the nicol is held between the calcite plate and the observer’s eye it can be so adjusted that only one image is seen. If now the nicol is revolved 90° the first image will disappear and the other image alone will be seen. Footnote 10: In some cases a peculiar form of double refraction does take place parallel to this direction, as in the circular polarization of quartz and cinnabar; but in very thin sections these results are not noticed and can be disregarded. Footnote 11: A. J. Moses, _Characters of Crystals_, pp. 98, 99. Footnote 12: The terms axes of elasticity are commonly used for these principal vibration directions in text-books on petrography. Footnote 13: 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. Footnote 14: