P = + in dir^n of plagihedral faces. J. Herschel. Q = Plane of Pol^n. twisted. Biot R = Fringes obliterated by stopping light from one edge or interposing a glass. Young. Arago. S = Ratios not reconcilable. _Irrationality_. Blair. T = Fresnel. U = Law exp. by surface of 4 dim^s. V = Optical classification of crystals. Brewster. W = Newt. Malus. Ray pol. in _principal plane_ of Rhomb.; and perp. to it. X = Brews. Biot. Ray pol. in plane bisecting ang. at axis; and perp. to it. Y = Brews. Ray pol. paral. to axis. Z = Malus. Ray pol. in plane of Refl. for _given angle_. a1 = Malus. Ray partially pol. in plane perp. to plane of Reflection. b1 = None Refl^d. if tan. ang. equal Refr. R^o. Brewster. c1 = Tint is as sq. of sin. Biot. d1 = Tint is as sin. α sin. β. Brewster. Biot. Lemniscates. J. Herschel. e1 = * By interf. of resolved undul^ns. of 2 rays circularly pol^d. in opp. directions. * Fresnel. f1 = * By interf. of resolved undul^ns. of 2 rays elliptically pol^d. in opp. directions. * Airy. g1 = * By interf. of rays from edges. Young. h1 = * Refl. produced by spherical undul^ns. i1 = * Refr. produced by spherical undul^ns. of diff. vel. for diff. colour. j1 = † Explanation imperfect. k1 = * Refr. produced by curved surf. undul^ns. l1 = * Pol^n. being prod. by resolution of transv^e undul^ns. m1 = * Polarization being produced by resolution of transverse undulations. n1 = * Undul^ns. being com^d. acc. to laws of elastic bodies. o1 = * Undul^ns. being com^d. acc. to a certain hypothesis. p1 = * Impossible formulæ being interpreted by analogy. q1 = * By interf. of resolved parts of transverse undul^ns. r1 = * Same hypothesis explains separation of rays in axis and oblique. † Explanation imperfect. * Maccullagh. s1 = † Explan. wanting. t1 = * By interf. of rays from all parts. * Young. * Fresnel. u1 = * By interf. of undul^ns. from all parts. * Fraunhofer. v1 = * By interf. of rays from striæ. * Young. w1 = * By interf. of undul^ns. from two surfaces. * Young. x1 = * Huyghens. Reflection and Refraction are propagation of undulations. y1 = * Young. * Fresnel. Polarization in crystals is transverse undulations. z1 = * Fresnel. Polarization in Reflection and Refraction is transverse undulations. A1 = * Fresnel. * Arago. Dipolarized Colours are produced by interference of Rays polarized in same plane; length of undulation being different for different colours. B1 = * Young. * Fresnel. Colours of Fringes, Gratings, Striæ, thick Plates, thin Plates &c. are produced by interference of undulations; length of undulation being different for different colours. C1 = * Undulations being propagated by the uniform elasticity of each medium. D1 = * Undul^ns. prop. by el^y. of medium diff. in 2 diff. dir^ns, (_axis of crystal._) E1 = * Undul^ns. being prop. by elasticity of med. diff. in 3 diff. directions (_axes_). F1 = Young. Reflection and double Refraction are propagation of undulations by crystalline elasticity. G1 = * Fresnel. Double Refr. and Pol.