TACINVARIANT—TETRAHEDROID. 
136 
numerative geometry, xi, 286; theorems of squares, xi, 299—313; theory of numbers, xi, 316; 
concomitants of ternary cubic, xi, 345—7; literal, for binary quantics, otherwise a partition table, 
xi, 357—64; for binary sextic, xi, 372—6, 377—88; covariantive, xi, 409—10; Pliicker’s equations, 
xi, 472; of Gauss, xi, 545; symmetric functions of roots of an equation, xn, 263—72, xm, 
272—4, 288; non-unitary partition, xn, 273—4; seminvariant, xn, 275—89; ortliomorphosis of 
circle into parabola, xn, 336; of groups, orders two to twelve, xn, 643—56; Wallis’s investigation for 
jr, xm, 23—5; quadrinvariant and cubinvariant of quadri-quadric function, xm, 68 ; partitions of 
a polygon, xm, 95, 112; theory of rational transformation, xm, 116; substitution groups for two 
to eight letters, xm, 118—49; corrected seminvariant for weights 11 and 12, xm, 217—23; of 
conjugates, xm, 303; seminvariants and symmetric functions, xm, 311, 313, 331—2; of pure 
reciprocals to weight 8, xm, 333—5; report of British Association committee on Pellian equation, 
and tables, xm, 430—67; omega and theta functions, xm, 558—9. 
Tacinvariant: the term, iv, 607. 
Tac-locus: in singular solutions, vm, 533. 
Tacnode: defined, n, 28—32, v, 266. 
Tactic: and algebra, v, 293—4; the term, xi, 443; (see also Arrangements, Groups). 
Tactinvariant: of two quantics, n, 320 ; the term, v, 305. 
Tactions: analytical solution, hi, 255—7; formulae, iv, 510—2; Casey’s equation, vi, 543; and trizomal 
curves, vi, 575; problem of, xm, 150—69. 
Tait, P. G.: arrangements, x, 245 ; quaternions, xn, 303, 475, xm, 541—4 ; finite differences, xn, 412. 
Talbot, W. H. F. : curve of, iv, 123. 
Tamisage : Sylvester’s theory of, xi, 409—10. 
Tangential: defined, n, 558; of a curve, iv, 188. 
Tangent Lines: and surface, xi, 630, 632—4. 
Tangent Omals: vi, 467—9. 
Tangent Planes: and surface, xi, 630, 632—4. 
Tangents: and two-dimensional geometry, n, 575; inflexional and chief, vm, 157, 294; singular of a 
quartic, x, 603; in Ency. Brit., xi, 564—5, 579—80; (see also Bitangents). 
Tannery, J. : linear differential equations, xn, 394. 
Tantipartite: the term, i, 584, n, 517, iv, 464, 604. 
Taylor, C.: locus in piano problem, vn, 599; general theory of surfaces, xn, 42—4. 
Taylor, H. M. : inversion, ix, 18; partitions of a polygon, xm, 93, 112. 
Taylor’s Theorem: Lagrange’s demonstration, vm, 493—5, 519 ; note on, vm, 524. 
Tchebycheff, P.: theory of numbers, xi, 616. 
Terminology: recent mathematical, iv, 594—608. 
Terms: in symmetrical determinant, ix, 185—90. 
Ternary: the term, iv, 604, vi, 464. 
Ternary Cubics: relation between two, iv, 79—81; memoir on quantics, iv, 325—41; form problem, 
vn, 548; the 34 concomitants, xi, 342—56; canonical form, xi, 343. 
Ternary Quadratics: resultant of three, iv, 349—58. 
Ternary Quadrics: and involution, xm, 350—3. 
Ternary Quantics : and bitangents of plane curve, iv, 188; involution, v, 301—9. 
Tetrads : the term, xn, 590; systems of, xm, 425—9. 
Tetrahedra: reciprocals, hi, 7; axial systems, v, 531—9; note on, v, 557—9; and cubic surfaces, 
vn, 607; correspondence of points in relation to two, vm, 200—8; Steiner’s surface, ix, 1—12; 
in perspective, ix, 209—10; automorphic function for, xi, 169, 179—83, 184, 212—6. 
Tetrahedral Surfaces : vn, 48—53, 54—65. 
Tetrahedroid: and wave surface, i, 302—5, 587; 16-nodal quartic surfaces, v, 431—7 ; the term, vi, 
21, x, 252-; and scrolls, vn, 245; as particular case of 16-nodal quartic surface, x, 437—40.