Chapter III: Appendix

Brief Scheme of Classification into Systems by Optical
Determinations.—Tables of Double Refraction (maximum) and
Indices of Refraction (mean).—Diagram, showing relation between
strength of double refraction, interference colors and
thickness of section.—Order of Consolidation of the Constituent
Minerals in Plutonic Rocks.—Optical Scheme with Special
Introduction. 141

INDEX. 149

CONVENTIONS AND ABBREVIATIONS.

a = The assumed direction of the ether vibrations of the fastest ray.

b = The assumed direction of the ether vibrations of the ray with intermediate velocity.

c = The assumed direction of the ether vibrations of the slowest ray.

In some American Text-Books (by Iddings, Winchell & Phillips) a = X, b = Y and c = Z.

a′ = The assumed direction of the ether vibrations of the faster ray in the given section.[1]

c′ = The assumed direction of the ether vibrations of the slower ray in the given section.

(+) = Optical character positive.

(−) = Optical character negative.

|| = Parallel to.

(γ − α) = The difference between the indices of refraction of the slowest and fastest rays, respectively, transmitted by the crystal, and indicates in decimals the relative strength of the double refraction.

_n′_ = The mean index of refraction;[2] hence

= (α + β + γ)/3 or (ε + 2ω)/3.

α = Index of refraction of the fastest ray.

γ = Index of refraction of the slowest ray.

_a_, _b_ and _ć_ relate to the crystallographic axes commonly represented by these letters.

2_E_ = the apparent axial angle measured in air, 2_V_ being the true angle.

_Bx_{a·}_ = The acute bisectrix.

_Bx_{c·}_ = The obtuse bisectrix.

_Ax. pl._ = The axial plane, _i. e._, the plane containing the two “optic axes.”

ELONGATION relates to the appreciable extension often shown by the crystal section. A crystal of long prismatic habit, cut about parallel to the _ć_ axis, would show marked elongation; while a tabular crystal (like mica) would show elongation if cut at right angles to the tabular faces. At times, of course, no elongation is appreciable, as in the case of granular or broken crystals or where the cross-section is essentially square or octagonal. Very often the relation of the cleavage is given to the elongation and also to the directions a′ and c′, which makes it possible to test for a′ and c′ even when no marked elongation can be observed.

MINERALS IN ROCK SECTIONS.