115 
OBLIQUITY-ORTHOTOMIC. 
Obliquity: the term, xiii, 234. 
Observations (see Orbits, Planet’s Orbit, Solar Eclipse). 
Octacron: enumeration of polyhedra, v, 38—44. 
Octad : the term, I, 586, vn, 133, 152, xn, 590. 
Octadic-quartic Surfaces: x, 51. 
Octagon: theorem of eight points on a conic, vm, 92—4. 
Octahedral Function : xr, 128—9. 
Octahedron : axial system, v, 531—9 ; automorpliic function for, xi, 169, 179—83, 212—6. 
Octaves : elliptic functions, r, 127, 586; imaginaries, i, 301. 
Octavic Surface : vm, 401—3. 
Octic Function : and Abelian function, xi, 483. 
Octics: and twisted cubics, xn, 310. 
Octic Surface : on a sibi-reciprocal, x, 252—5 ; (see also Surfaces). 
Octo-dianome: the term, vn, 134. 
Octo-hexahedron: the term, x, 328. 
Odd Branch of Curve: x, 36. 
Off-planes: the term, vi, 330, 577, 583—5. 
Off-points : the term, vi, 330, 338, 577, 583—5. 
Olbers, W.: orbits of asteroids, ix, 177. 
Olivier, T.: conics inscribed in quadric surface, I, 557. 
Omal: the word, vi, 194, 463, 467—9. 
Omega Functions: the term, xi, 453; note on Smith’s memoir, xm, 558—9. 
Omphali: the term, vm, 326. 
Operandator: defined, in, 242. 
Operations: and substitutions, xm, 530. 
Operators: differential, vii, 8 ; and seminvariants, xm, 322—32; MacMahon’s multilinear, xm, 399. 
Optics: MacCullagh’s theorem in polarized light, iv, 12—20; geometrical construction in, x, 28. 
Orbits: Jacobi’s canonical formulae for disturbed motion, in, 76—7; reduction to fixed plane, iii, 
91—6; variation in plane of planet’s, hi, 516—8; central forces problem, iv, 516—21; position 
of, in planetary theory, vn, 541—5 ; of asteroid, and Newcomb, ix, 176—7; Hamiltonian equations 
of central, x, 613; Sylvester’s work at, xm, 47; (see also Planetary Theory, Planet’s Orbit). 
Order: of system of equations, I, 457—61, 589; of qualities, defined, n, 221; of curve, n, 569—83, 
xi, 462 ; in abstract geometry, defined, vi, 463; of curve and surface, xi, 629. 
Ordinary Point for Differential Equations: xn, 394. 
Oriani, B.: elliptic motion, hi, 474, iv, 528. 
Orr, W. McF.: tetrads of circles, xm, 425. 
Orthocentre: the term, xm, 550. 
Orthogonal Surfaces: vm, 269—91, 292, 569—70; Smith’s Prize dissertation, vm, 558—63. 
Orthogonal Surfaces and Curvature, Memoir on: vm, 292—315; introductory, vm, 292—3 ; curvature 
of surfaces, vm, 293—300; conormal correspondence of vicinal surfaces, vm, 301—8; condition that 
the two surfaces may belong to orthogonal system, vm, 309—11; family of surfaces, vm, 312—5. 
Orthomorphic Transformation (see Orthomorphosis). 
Orthomorphosis: of circle into parabola, v, 618, xn, 328—36; of a circle into itself, xm, 20; 
general theory, xm, 170—90; some problems, xm, 191—205; note on theory, xm, 418—24; (see 
also Conformal Representation). 
Orthotomic: the term, ix, 13. 
Orthotomic Circles: and polyzomal curves, vi, 501; and Jacobians, vi, 568. 
Orthotomic Curve: of a system of lines in a plane, xm, 346—7. 
Orthotomic Surfaces: in Ency. Brit., xi, 637—8. 
15—2