The Project Gutenberg EBook of The Elements of non-Euclidean Geometry, by Julian Lowell Coolidge This eBook is for the use of anyone anywhere at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this eBook or online at www.gutenberg.org Title: The Elements of non-Euclidean Geometry Author: Julian Lowell Coolidge Release Date: August 20, 2008 [EBook #26373] Language: English Character set encoding: ISO-8859-1 *** START OF THIS PROJECT GUTENBERG EBOOK NON-EUCLIDEAN GEOMETRY *** Produced by Joshua Hutchinson, David Starner, Keith Edkins and the Online Distributed Proofreading Team at http://www.pgdp.net THE ELEMENTS OF NON-EUCLIDEAN GEOMETRY BY JULIAN LOWELL COOLIDGE Ph.D. ASSISTANT PROFESSOR OF MATHEMATICS IN HARVARD UNIVERSITY OXFORD AT THE CLARENDON PRESS 1909 PREFACE The heroic age of non-euclidean geometry is passed. It is long since the days when Lobatchewsky timidly referred to his system as an ‘imaginary geometry’, and the new subject appeared as a dangerous lapse from the orthodox doctrine of Euclid. The attempt to prove the parallel axiom by means of the other usual assumptions is now seldom undertaken, and those who do undertake it, are considered in the class with circle-squarers and searchers for perpetual motion– sad by-products of the creative activity of mo dern science. In this, as in all other changes, there is subject both for rejoicing and regret. It is a satisfaction to a writer on non-euclidean geom etry that he may proceed at once to his subject, without feeling any need to justify himself, or, at least, any more need than any other who adds to our supply of books. On the other hand, he will miss the stimulus that comes to one who feels that he is bringing out something entirely new and strange. The subject of non-euclidean geome- try is, to the mathematician, quite as well established as any other branch of mathematical science; and, in fact, it may lay claim to a decidedly more solid basis than some branches, such as the theory of assemblages, or the analysis situs. Recent books dealing with non-euclidean ge ometry fall naturally into two classes. In the one we find the works of Killing, Liebmann, and Manning, 1 who wish to build up certain clearly conceived geometrical systems, and are careless of the details of the foundations on which all is to rest. In the other category are Hilbert, Vablen, Veronese, and the authors of a goodly number of articles on the foundations of geometry. These writers deal at length with the consistency, significance, and logical independence of their assumptions, but do not go very far towards raising a superstructure on any one of the foundations suggested. The present work is, in a measure, an attempt to unite the two tendencies. The author’s own interest, be it stated at the outset, lies mainly in the fruits, rather than in the roots; but the day is past when the matter of axioms may be dismissed with the remark that we ‘make all of Euclid’s assumptions except the one about parallels’. A subject like ours must be built up from explicitly stated assumptions, and nothing else. The author would have preferred, in the first chapters, to start from some system of axioms already published, had he been familiar with any that seemed to him suitable to establish simultaneously the euclidean and the principal non-euclidean systems in the way that he wished. The system of axioms here used is decidedly more c umbersome than some others, but leads to the desired goal. There are three natural approaches to non-euclidean geometry. (1) The elementary geometry of point, line, and distance. This method is developed in the opening chapters and is the most obvious. (2) Projective geometry, and the theory of transformation groups. This method is not taken up until Chapter XVIII, not b e cause it is one whit less important than the first, but because it seemed better not to interrupt the natural course of the narrative 1 Detailed references given later 1 by interpolating an alternative beginning. (3) Differential geometry, with the concepts of distance-e leme nt, extremal, and space constant. This method is explained in the last chapter, XIX. The author has imp ose d upon himself one or two very definite limitations. To begin with, he has not gone beyond three dimensions. This is because of his feeling that, at any rate in a first study of the subject, the gain in gener- ality obtained by studying the geometry of n-dimensions is more than offset by the loss of clearness and naturalness. Secondly, he has confined himself, al- most exclusively, to what may be called the ‘classical’ non-euclidean systems. These are much more closely allied to the euclidean system than are any oth- ers, and have by far the most historical importance. It is also evident that a system which gives a simple and clear interpretation of ternary and quaternary orthogonal substitutions, has a totally different sort of mathematical signifi- cance from, let us say, one whose points are determined by numerical values in a non-archimedian number system. Or again, a non-euclidean plane which may be interpreted as a surface of constant total curvature, has a more las ting geometrical importance than a non-desarguian plane that cannot form part of a three-dimensional space. The majority of material in the present work is, naturally, old. A reader, new to the subject, may find it wiser at the first reading to omit Chapters X, XV, XVI, XVIII, and XIX. On the other hand, a reader already somewhat familiar with non-euclidean geometry, may find his greatest interest in Chap- ters X and XVI, which contain the substance of a number of recent papers on the extraordinary line geometry of non-euclidean space. Mention may also be made of Chapter XIV which contains a number of neat formulae relative to areas and volumes published many years ago by Professor d’Ovidio, which are not, perhaps, very familiar to English-speaking readers, and Chapter XIII, where Staude’s string construction of the ellipsoid is extended to non-euclidean space. It is hoped that the introduction to non-euclidean differential geometry in Chapter XV may prove to be more comprehensive than that of Darboux, and more comprehensible than that of Bianchi. The author takes this opportunity to thank his colleague, Assistant-Professor Whittemore, who has read in manuscript Chapters XV and XIX. He would also offer affectionate thanks to his former teachers, Professor Eduard Study of Bonn and Professor Corrado Segre of Turin, and all others who have aided and encouraged (or shall we say abetted?) him in the present work. 2 TABLE OF CONTENTS CHAPTER I FOUNDATION FOR METRICAL GEOMETRY IN A LIMITED REGION Fundamental assumptions and definitions . . . . . . . . . . . . . . . . . . . 9 Sums and differences of distances . . . . . . . . . . . . . . . . . . . . . . . . 10 Serial arrangement of points on a line . . . . . . . . . . . . . . . . . . . . . 11 Simple descriptive properties of plane and space . . . . . . . . . . . . . . . 14 CHAPTER II CONGRUENT TRANSFORMATIONS Axiom of continuity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Division of distances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Measure of distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Axiom of congruent transformations . . . . . . . . . . . . . . . . . . . . . . 21 Definition of angles, their properties . . . . . . . . . . . . . . . . . . . . . . 22 Comparison of triangles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Side of a triangle not greater than sum of other two . . . . . . . . . . . . . 26 Comparison and measurement of angles . . . . . . . . . . . . . . . . . . . . 28 Nature of the congruent group . . . . . . . . . . . . . . . . . . . . . . . . . 29 Definition of dihedral angles, their properties . . . . . . . . . . . . . . . . . 29 CHAPTER III THE THREE HYPOTHESES A variable angle is a continuous function of a variable distance . . . . . . . 31 Saccheri’s theorem for isosceles birectangular quadrilaterals . . . . . . . . . 33 The existence of one rectangle implies the existence of an infinite number . 34 Three assumptions as to the sum of the angles of a right triangle . . . . . . 34 Three assumptions as to the sum of the angles of any triangle, their categorical nature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Definition of the euclidean, hyperbolic, and elliptic hypotheses . . . . . . . 35 Geometry in the infinitesimal domain obeys the euclidean hypothesis . . . . 37 CHAPTER IV THE INTRODUCTION OF TRIGONOMETRIC FORMULAE Limit of ratio of opposite sides of diminishing isosceles quadrilateral . . . . 38 Continuity of the resulting function . . . . . . . . . . . . . . . . . . . . . . 40 Its functional equation and solution . . . . . . . . . . . . . . . . . . . . . . 40 Functional equation for the cosine of an angle . . . . . . . . . . . . . . . . . 43 3 Non-euclidean form for the pythagorean theorem . . . . . . . . . . . . . . . 43 Trigonometric formulae for right and oblique triangles . . . . . . . . . . . . 45 CHAPTER V ANALYTIC FORMULAE Directed distances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Group of translations of a line . . . . . . . . . . . . . . . . . . . . . . . . . 49 Positive and negative directed distances . . . . . . . . . . . . . . . . . . . . 50 Coordinates of a point on a line . . . . . . . . . . . . . . . . . . . . . . . . 50 Coordinates of a point in a plane . . . . . . . . . . . . . . . . . . . . . . . . 50 Finite and infinitesimal distance formulae, the non-euclidean plane as a sur- face of constant Gaussian curvature . . . . . . . . . . . . . . . . . 51 Equation connecting direction cosines of a line . . . . . . . . . . . . . . . . 53 Coordinates of a point in space . . . . . . . . . . . . . . . . . . . . . . . . . 54 Congruent transformations and orthogonal substitutions . . . . . . . . . . . 55 Fundamental formulae for distance and angle . . . . . . . . . . . . . . . . . 56 CHAPTER VI CONSISTENCY AND SIGNIFICANCE OF THE AXIOMS Examples of geometries satisfying the assumptions made . . . . . . . . . . 58 Relative independence of the axioms . . . . . . . . . . . . . . . . . . . . . . 59 CHAPTER VII THE GEOMETRIC AND ANALYTIC EXTENSION OF SPACE Possibility of extending a segment by a definite amount in the euclidean and hyperbolic cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Euclidean and hyperbolic space . . . . . . . . . . . . . . . . . . . . . . . . . 62 Contradiction arising under the elliptic hypothesis . . . . . . . . . . . . . . 62 New assumptions identical with the old for limited region, but permitting the extension of every segment by a definite amount . . . . . . . . . . 63 Last axiom, free mobility of the whole system . . . . . . . . . . . . . . . . . 64 One to one correspondence of point and coordinate set in euclidean and hy- perbolic cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Ambiguity in the elliptic case giving rise to elliptic and spherical geometry 65 Ideal elements, extension of all spaces to be real continua . . . . . . . . . . 67 Imaginary elements geometrically defined, extension of all spaces to be perfect continua in the complex domain . . . . . . . . . . . . . . . . . . . 68 Cayleyan Absolute, new form for the definition of distance . . . . . . . . . 70 Extension of the distance concept to the complex domain . . . . . . . . . . 71 Case where a straight line gives a maximum distance . . . . . . . . . . . . . 73 4 CHAPTER VIII THE GROUPS OF CONGRUENT TRANSFORMATIONS Congruent transformations of the straight line . . . . . . . . . . . . . . . . 76 ,, ,, ,, hyperbolic plane . . . . . . . . . . . . . . 76 ,, ,, ,, elliptic plane . . . . . . . . . . . . . . . . 77 ,, ,, ,, euclidean plane . . . . . . . . . . . . . . 78 ,, ,, ,, hyperbolic space . . . . . . . . . . . . . . 78 ,, ,, ,, elliptic and spherical space . . . . . . . . 80 Clifford parallels, or paratactic lines . . . . . . . . . . . . . . . . . . . . . . 80 The groups of right and left translations . . . . . . . . . . . . . . . . . . . . 80 Congruent transformations of euclidean space . . . . . . . . . . . . . . . . . 81 CHAPTER IX POINT, LINE, AND PLANE TREATED ANALYTICALLY Notable points of a triangle in the non-euclidean plane . . . . . . . . . . . . 83 Analoga of the theorems of Menelaus and Ceva . . . . . . . . . . . . . . . . 85 Formulae of the parallel angle . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Equations of parallels to a given line . . . . . . . . . . . . . . . . . . . . . . 88 Notable points of a tetrahedron, and resulting desmic configurations . . . . 89 Invariant formulae for distance and angle of skew lines in line coordinates . 91 Criteria for parallelism and parataxy in line coordinates . . . . . . . . . . . 93 Relative moment of two directed lines . . . . . . . . . . . . . . . . . . . . . 95 CHAPTER X THE HIGHER LINE GEOMETRY Linear complex in hyperbolic space . . . . . . . . . . . . . . . . . . . . . . . 96 The cross, its coordinates . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 The use of the cross manifold to interpret the geometry of the complex plane 98 Chain, and chain surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 Hamilton’s theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Chain congruence, synectic and non-synectic congruences . . . . . . . . . . 100 Dual coordinates of a cross in elliptic case . . . . . . . . . . . . . . . . . . . 102 Condition for parataxy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Clifford angles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Chain and strip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 Chain congruence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 CHAPTER XI THE CIRCLE AND THE SPHERE Simplest form for the equation of a circle . . . . . . . . . . . . . . . . . . . 109 Dual nature of the curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 Curvature of a circle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Radical axes, and centres of similitude . . . . . . . . . . . . . . . . . . . . . 112 Circles through two points, or tangent to two lines . . . . . . . . . . . . . . 112 5 Spheres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 Poincar´e’s sphere to sphere transformation from euclidean to non-euclidean space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 CHAPTER XII CONIC SECTIONS Classification of conics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 Equations of central conic and Absolute . . . . . . . . . . . . . . . . . . . . 119 Centres, axes, foci, focal lines, directrices, and director points . . . . . . . . 120 Relations connecting distances of a point from foci, directrices, &c., and their duals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 Conjugate and mutually perpendicular lines through a centre . . . . . . . . 124 Auxiliary circles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Normals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Confocal and homothetic conics . . . . . . . . . . . . . . . . . . . . . . . . 128 Elliptic coordinates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 CHAPTER XIII QUADRIC SURFACES Classification of quadrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 Central quadrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 Planes of circular section and parabolic section . . . . . . . . . . . . . . . . 133 Conjugate and mutually perpendicular lines through a centre . . . . . . . . 134 Confocal and homothetic quadrics . . . . . . . . . . . . . . . . . . . . . . . 135 Elliptic coordinates, various forms of the distance elem ent . . . . . . . . . . 135 String construction for the ellipsoid . . . . . . . . . . . . . . . . . . . . . . 140 CHAPTER XIV AREAS AND VOLUMES Amplitude of a triangle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Relation to other parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 Limiting form when the triangle is infinitesimal . . . . . . . . . . . . . . . . 146 Deficiency and area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Area found by integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 Area of circle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 Area of whole elliptic or spherical plane . . . . . . . . . . . . . . . . . . . . 150 Amplitude of a tetrahedron . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 Relation to other parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 Simple form for the differential of volume of a tetrahedron . . . . . . . . . . 152 Reduction to a single quadrature of the problem of finding the volume of a tetrahedron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 Volume of a cone of revolution . . . . . . . . . . . . . . . . . . . . . . . . . 155 Volume of a sphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 Volume of the whole of elliptic or of spherical space . . . . . . . . . . . . . 156 6 CHAPTER XV INTRODUCTION TO DIFFERENTIAL GEOMETRY Curvature of a space or plane curve . . . . . . . . . . . . . . . . . . . . . . 157 Analoga of direction cosines of tangent, principal normal, and binormal . . 158 Frenet’s formulae for the non-euclidean case . . . . . . . . . . . . . . . . . . 159 Sign of the torsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 Evolutes of a space c urve . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 Two fundamental quadratic differential forms for a s urface . . . . . . . . . 163 Conditions for mutually conjugate or perpendicular tangents . . . . . . . . 164 Lines of curvature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Dupin’s theorem for triply orthogonal systems . . . . . . . . . . . . . . . . 166 Curvature of a curve on a surface . . . . . . . . . . . . . . . . . . . . . . . . 168 Dupin’s indicatrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 Torsion of asymptotic lines . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 Total relative curvature, its relation to Gaussian curvature . . . . . . . . . 171 Surfaces of zero relative curvature . . . . . . . . . . . . . . . . . . . . . . . 172 Surfaces of zero Gaussian curvature . . . . . . . . . . . . . . . . . . . . . . 173 Ruled surfaces of zero Gaussian curvature in elliptic or spherical space . . . 174 Geodesic curvature and geodesic lines . . . . . . . . . . . . . . . . . . . . . 175 Necessary conditions for a minimal surface . . . . . . . . . . . . . . . . . . 178 Integration of the resulting differential equations . . . . . . . . . . . . . . . 179 CHAPTER XVI DIFFERENTIAL LINE-GEOMETRY Analoga of Kummer’s coefficients . . . . . . . . . . . . . . . . . . . . . . . . 182 Their fundamental relations . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 Limiting points and focal points . . . . . . . . . . . . . . . . . . . . . . . . 185 Necessary and sufficient conditions for a normal congruence . . . . . . . . . 188 Malus-Dupin theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Isotropic congruences, and congruences of normals to surfaces of zero curva- ture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Spherical representation of rays in elliptic space . . . . . . . . . . . . . . . . 193 Representation of normal congruence . . . . . . . . . . . . . . . . . . . . . . 194 Isotropic congruence represented by an arbitrary function of the complex variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 Special examples of this representation . . . . . . . . . . . . . . . . . . . . . 197 Study’s ray to ray transformation which interchanges parallelism and para- taxy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 Resulting interchange among the three special types of congruence . . . . . 199 7 CHAPTER XVII MULTIPLY CONNECTED SPACES Repudiation of the axiom of free mobility of space as a whole . . . . . . . . 200 Resulting possibility of one to many correspondence of points and coordinate sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 Multiply connected euclidean planes . . . . . . . . . . . . . . . . . . . . . . 202 Multiply connected euclidean spaces, various types of line in them . . . . . 203 Hyperb olic case little known; relation to automorphic functions . . . . . . . 205 Non-existence of multiply connected elliptic planes . . . . . . . . . . . . . . 207 Multiply connected elliptic spaces . . . . . . . . . . . . . . . . . . . . . . . 208 CHAPTER XVIII THE PROJECTIVE BASIS OF NON-EUCLIDEAN GEOMETRY Fundamental notions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210 Axioms of connexion and separation . . . . . . . . . . . . . . . . . . . . . . 210 Projective geometry of the plane . . . . . . . . . . . . . . . . . . . . . . . . 211 Projective geometry of space . . . . . . . . . . . . . . . . . . . . . . . . . . 212 Projective scale and cross ratios . . . . . . . . . . . . . . . . . . . . . . . . 216 Projective coordinates of points in a line . . . . . . . . . . . . . . . . . . . . 220 Linear transformations of the line . . . . . . . . . . . . . . . . . . . . . . . 221 Projective coordinates of points in a plane . . . . . . . . . . . . . . . . . . . 221 Equation of a line, its coordinates . . . . . . . . . . . . . . . . . . . . . . . 222 Projective coordinates of points in space . . . . . . . . . . . . . . . . . . . . 222 Equation of a plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 Collineations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 Imaginary elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 Axioms of the congruent collineation group . . . . . . . . . . . . . . . . . . 226 Reappearance of the Absolute and previous metrical formulae . . . . . . . . 229 CHAPTER XIX THE DIFFERENTIAL BASIS FOR EUCLIDEAN AND NON-EUCLIDEAN GEOMETRY Fundamental assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 Coordinate system and distance elements . . . . . . . . . . . . . . . . . . . 232 Geodesic curves, their differential equations . . . . . . . . . . . . . . . . . . 233 Determination of a geodesic by two near points . . . . . . . . . . . . . . . . 234 Determination of a geodesic by a point and direction cosines of tangent thereat 234 Definition of angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 Axiom of congruent transformations . . . . . . . . . . . . . . . . . . . . . . 235 Simplified expression for distance element . . . . . . . . . . . . . . . . . . . 236 Constant curvature of geodesic surfaces . . . . . . . . . . . . . . . . . . . . 237 Introduction of new coordinates; integration of equations of geodesic . . . . 239 Reappearance of familiar distance formulae . . . . . . . . . . . . . . . . . . 240 Recapitulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 8 [...]... ipotetico deduttivo,’ Memorie della R Accademia delle Scienze di Torino, Serie 2, vol xlix, 1899, introduces motion alone, as does Padoa, ‘Un nuovo sistema di de nizioni per la geometria euclidea,’ Periodico di matematica, Serie 3, vol i, 1903 Vahlen, Abstrakte Geometrie, Leipzig, 1905, uses line and separation Peano, ‘La geometria basata sulle idee di punto e di distanza,’ Atti della R Accademia di Torino,... In other words, there is no distance which may not be extended in either direction It is, however, fundamentally important to notice that we have made no assumption as to the magnitude of the amount by which a distance may be so extended; we have merely premised the existence of such extension We shall make the concept of extension more explicit by the following de nitions De nition The assemblage... point of one side of a triangle, but no point of a second side, it must contain a point of the third Theorem 31 If a line in the plane of a triangle contain a point of one side of the triangle and no point of a second side, it must contain a point of the third side De nition If a point within the segment of two given points be in a given plane, those points shall be said to be on opposite sides of the... shall be called the exterior angle of the two half-lines The de nitions for sides and vertex shall be as before If no mention be made of the words interior or exterior we shall understand by the word angle, interior angle Notice that, by our de nitions, the sides are a part of the interior, but not of the exterior angle Let the reader also show that if a half-line of an interior angle be taken, the other... The de nition of sides and vertex shall be as before De nition Two collinear, but not identical, half-lines of common bound shall be said to be opposite De nition The assemblage of all half-lines having as bound the common bound of two opposite half-lines, and lying in any half-plane bounded by the line of the latter, shall be called an angle of the two half-lines in that plane The de nitions of sides... theorem Theorem 18 The plane determined by three vertices of a triangle is identical with that determined by two of their number and any other point of the line of either of the remaining sides Theorem 19 Any one of the three points determining a plane may be replaced by any other point of the plane, not collinear with the two remaining determining points Theorem 20 A plane may be determined by any three... form given 13 Theorem 16 If a line contain a point of one side of a triangle and one of either extension of a second side, it will contain a point of the third side De nition The assemblage of all points of all lines determined by the vertices of a triangle and all points of the opposite sides shall be called a plane It should be noticed that in de ning a plane in this manner, the vertices of the triangle... shall be those non-re-entrant angles whose vertices are the vertices of the triangle, and whose sides include the sides of the triangle De nition The angle between a half-line including one side of a triangle, and bounded at a chosen vertex, and the opposite of the other half-line which goes to make the angle of the triangle at that vertex, shall be called an exterior angle of the triangle Notice that... are not to be confused with the exterior angles of their respective sides Theorem 16 If two triangles be so related that the sides of one are congruent to those of the other, the same holds for the angles This is an immediate result of 11 The meanings of the words opposite and adjacent as applied to sides and angles of a triangle are immediately evident, and need not be de ned There can also be no ambiguity... de ned There can also be no ambiguity in speaking of sides including an angle Theorem 17 Two triangles are congruent if two sides and the included angle of one be respectively congruent to two sides and the included angle of the other The truth of this is at once evident when we recall the de nition of congruent angles, and 12 Theorem 18 If two sides of a triangle be congruent, the opposite angles are . English-speaking readers, and Chapter XIII, where Staude’s string construction of the ellipsoid is extended to non-euclidean space. It is hoped that the introduction to non-euclidean differential. vol. v, 1904, use segment and order. Pieri, ‘Della geometria elementare come sistema ipotetico dedut- tivo,’ Memorie della R. Accademia del le Scienze di Torino, Serie 2, vol. xlix, 1899, introduces motion. words, there is no distance which may not be extended in either direction. It is, however, fundamentally important to notice that we have made no assumption as to the magnitude of the amount