. The optical indicatrix and the transmission of light in crystals. i 1 Pliilos. Miiii~iii< 1888, ser. 5, vol. 2G, p. 628: FAectrnity and Magnidsm, byJ. Clerk Maxwell; Oxford, 1881, vol. 2, p. 40-i; Nature, 1890, vol. 42, p. 174. THE RAY-SURFACE. 33 2. The symmetry of the fay-surface. From the mode of construction, it is evident that the planes of symmetryof the indicatrix are also planes of s^^mmetry of the ray-surface. 3. The sections of the ray-surface by the planes of symmetry. The section of the ray-surface by each plane of symmetry consistsof a circle and an ellipse ; the radius of th
. The optical indicatrix and the transmission of light in crystals. i 1 Pliilos. Miiii~iii< 1888, ser. 5, vol. 2G, p. 628: FAectrnity and Magnidsm, byJ. Clerk Maxwell; Oxford, 1881, vol. 2, p. 40-i; Nature, 1890, vol. 42, p. 174. THE RAY-SURFACE. 33 2. The symmetry of the fay-surface. From the mode of construction, it is evident that the planes of symmetryof the indicatrix are also planes of s^^mmetry of the ray-surface. 3. The sections of the ray-surface by the planes of symmetry. The section of the ray-surface by each plane of symmetry consistsof a circle and an ellipse ; the radius of the circle is the inverse of thataxis of the indicatrix which is perpendicular to the plane of the section ;the ellipse is similar and similarly situated to the section of the indicatrixby the same plane. This may be proved as follows:— Let AOA, BOB, COC, be the principal axes of the indicatrix, and 0A= —» OB = —, 0C= —: it is required to determine the section ofa b c the ray-surface by one of the planes of symmetry, say AOC (Fig. G).. Fig. G. a. By considering a series of points in a small ring surrounding thepoint B on the indicatrix, it is seen that in the limit the point B itseUcorresponds, not to one ray, but to an infinity of rays, all lying in th^plnno AOC; for the axis OB is the normal of the indicatrix at B, andintersects perpendicularly all radii vectores of the indicatrix which lie in theplane AOC. Further, the length of the normal at 1) intercepted by eachof the rays is OB : hence the velocity of each ray corresponding to the point J3 is /r5-or /, and a circle of radius b, situated in the plane AOC,(JB is on the ray-surface (Fig 7c). 34 THE TRANSMISSION OF LIGHT IN CKYSTALS. b. For a point R on the indicatrix, lying in the plane A OC, the normalR Vof the indicatrix lies in the plane AOC, and is also normal at the pointR to the elliptic section ACAC: if OXr is perpendicular to RX and Or = p^, Or is by construction a radius vector of the ray-surface (Fig. G). I
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