46. DIAGRAM INDICATING THE CONSEQUENCES OF A COLLISION BETWEEN TWO EXTINCT SUNS 157 47. SPIRAL NEBULA IN THE CANES VENATICI 159 48. SPIRAL NEBULA IN THE TRIANGLE 161 49. THE GREAT NEBULA IN ANDROMEDA 163 50. RING-SHAPED NEBULA IN LYRA 164 51. CENTRAL PORTION OF THE GREAT NEBULA IN ORION 165 52. NEBULAR STRIÆ IN THE STARS OF THE PLEIADES 167 53. NEBULAR STRIÆ IN THE SWAN 169 54. NEBULA AND STAR RIFT IN THE SWAN, IN THE MILKY WAY 171 55. GREAT NEBULA NEAR RHO, IN OPHIUCHUS 172 56. STAR CLUSTER IN HERCULES 173 57. STAR CLUSTER IN PEGASUS 175 58. CONE-SHAPED STAR CLUSTER IN GEMINI 176 59. COMPARISON OF SPECTRA OF STARS OF CLASSES 2, 3, 4 185 60. COMPARISON OF SPECTRA OF STARS OF CLASSES 2, 3, 4 186 PREFACE When, more than six years ago, I was writing my _Treatise of Cosmic Physics_, I found myself confronted with great difficulties. The views then held would not explain many phenomena, and they failed in particular in cosmogonic problems. The radiation pressure of light, which had not, so far, been heeded, seemed to give me the key to the elucidation of many obscure problems, and I made a large use of this force in dealing with those phenomena in my treatise. The explanations which I tentatively offered could, of course, not claim to stand in all their detail; yet the scientific world received them with unusual interest and benevolence. Thus encouraged, I tried to solve more of the numerous important problems, and in the present volume I have added some further sections to the complex of explanatory arguments concerning the evolution of the Universe. The foundation to these explanations was laid in a memoir which I presented to the Academy of Sciences at Stockholm in 1900. The memoir was soon afterwards printed in the _Physikalische Zeitschrift_, and the subject was further developed in my _Treatise of Cosmic Physics_.