4. Luminosities, Colours, Diameters, Densities, Masses of the Stars.- d) The Diameters of the Stars.- 190. The Diameters of the Stars. Earlier Conceptions. Pioneer Work.- 191. Wilsing’s Investigations.- 192. Russell’s Method.- 193. Diameters from c2/T.- 194. Diameters from Radiometric Measurements.- 195. Kalmár’s Investigation.- 196. Interferometer Measurements at Mount Wilson.- 197. Varying Stellar Diameters.- 198. The Theoretical Investigations of M. Hamy.- 199. Danjon’s Interferometer-Method.- 200. The Companion of Sirius (Sirius B).- 201. Diameters from Scintillation-Observations.- 202. The Fallacy in S. Pokrowsky’s Method.- e) The Densities of the Stars.- 203. Densities of the Stars. Pioneer Work.- 204. Densities of Visual Binary Stars.- 205. The Ratio of Densities in Double Stars.- 206. Densities of Eclipsing Binaries. Methodical.- 207. Shapley’s Work.- 208. Parallaxes and Absolute Magnitudes of Eclipsing Binaries.- 209. Recent Statistics of the Eclipsing Binaries.- f) The Masses of the Stars.- 210 Methods of Deriving Stellar Masses.- 211. Are Derived Mass-Values Representative?.- 212. Historical Notes. Observational Evidences.- 213. Equipartition of Stellar Energy.- 214. Schlesinger’s and Baker’s Study on Spectroscopic Binaries.- 215. Ludendorff’s Researches on the Masses of Spectroscopic Binaries.- 216. Frequency of Stellar Masses for Different Spectral Classes.- 217. Sproul Determinations of Masses.- 218. Pitman’s Investigation.- 219. Statistics of accurately Determined Stellar Masses.- 220. Real and Apparent Masses.- 221. Seares’s Researches.- 222. Stellar Masses from Spectrographic Parallaxes.- 223. Masses of F—K Stars.- 224. Colour-Mass-Density Relation.- 225. von Zeipel’s Method.- 226. The Method of Freundlich and Heiskanen.- 227. Martens’s Method.- 228. Recent Work Concerning Masses of Spectroscopic Binaries.- 229. Preferential Values of Stellar Masses.- 230. Dwarf Nature of Spectroscopic Binaries.- 231. Discovery of Mass-Luminosity Relation.- 232. Eddington’s Mass-Luminosity Law and Cosmogonie Time-Scale.- 233. Discrepancies between Seares’s and Eddington’s Results.- 234. Jeans’s Theory.- 235. The Cosmogonic Time-Scale by Jeans and Smart.- 236. Brill’s Theory and Parallax-Method for Binaries.- 237. Mass-Reduction by Annihilation of Protons and Electrons.- 238. The Theory of Rabe.- 239. Convergence of Mass-Ratios with Increasing Age.- 240. Vogt’s Extension of Eddington’s Theory.- 241. Statistical Investigations Concerning the Mass-Ratio in Binaries.- 242. Theoretical Derivation of the Mass-Ratio in Double Stars.- 243. Lundmark’s and Luyten’s Differential Method.- 244. The Masses and Luminosities of the Eclipsing Binaries.- 245. The Upper Limit for the Stellar Masses.- 246. Relation between Stellar Mass and Proper Motion.- 247. Relation between Stellar Mass and Form of Orbits of Binary Stars.- 248. The Mass of the Orion Nebula.- 249. Planetary Nebulae.- 250. Mass of the Stellar System.- 251. The Masses and Mass-Ratios of Stellar Systems.- 252. The Angular Moments of Visual Binaries.- 253. The Origin of Binary Stars.- 254. Concluding Remarks.- Appendix I. Catalogue of Stars Brighter than 5m,00.- Appendix II. Catalogue of Stellar Diameters.- 5. Stellar Clusters.- a) Introductory Survey.- 1. The Significance of Clusters.- 2. Historical Notes on Clusters.- b) Classification, Number and Distribution.- 3. A Comparison of Galactic and Globular Clusters.- 4. The Number of Clusters.- 5. Classification of Galactic Clusters.- 6. Classification of Globular Clusters.- 7. Clusters in or near Obstructing Nebulosity.- 8. The Apparent Distribution of Globular Clusters.- 9. The Apparent Distribution of Galactic Clusters.- 10. Peculiarities in the Distribution of Galactic Clusters.- c) On the Spectral Composition of Clusters.- 11. Integrated Spectra of Globular Clusters.- 12. Stellar Types in Globular Clusters.- 13. On the Masses of Giant Stars.- 14. Spectra in Individual Galactic Clusters.- d) Variable Stars in Star Clusters.- 15. The Frequency and General Properties of Variable Stars in Clusters.- 16. A Summary of Known Variables.- e) The Distribution of Stars in Globular Clusters.- 17. Are Cluster Stars Arranged Spirally?.- 18. On the Laws of Distribution.- 19. Luminosity Curves for Stars in Clusters.- f) The Forms of Clusters.- 20. Definition and Difficulties.- 21. The Elongation of Messier 13.- 22. Ellipticity and Orientation of Globular Clusters.- 23. Some Peculiar Clusters.- 24. The Structure of Galactic Clusters.- g) The Transparency of Space.- 25. Early Investigations of Light Scattering.- 26. Blue Stars in Messier 13.- 27. Faint Blue Stars in the Milky Way.- 28. Colors in Other Distant Objects.- 29. Messier 5 and the Relative Speeds of Blue and Yellow Light.- h) The Distances and Dimensions of Clusters.- 30. The Photographic Period-Luminosity Curve.- 31. Distances of Globular Clusters Obtained from Cepheids and Bright Stars.- 32. Distances of Globular Clusters Obtained from Diameters and Integrated Magnitudes.- 33. A Working Catalogue of Galactic Clusters (Appendix B).- 34. Parallaxes of Galactic Clusters.- 35. Radial Velocities of Globular Clusters.- 36. Dimensions and Star Densities of Clusters.- i) Star Clusters in the Magellanic Clouds.- 37. Types of Clusters and Nebulae.- 38. The Globular Star Clusters.- 39. Distances of the Clouds.- 40. On the Relation of the Clusters to the Magellanic Clouds.- j) Dimensions of the Galaxy.- 41. Membership in the Galaxy.- 42. The System of Galactic Clusters.- 43. The Higher Systems of Globular Clusters.- 44. The Distance to the Galactic Center.- 45. On the Size and Structure of the Galaxy.- Appendix A. Catalogue of Globular Clusters.- Appendix B. Catalogue of Galactic Clusters.- 6. The Nebulae.- a) Introduction.- 1. Definition of Nebulae.- 2. Historical Notes.- 3. Bibliographical Notes.- 4. Classification and Units.- b) The Diffuse Nebulae.- 5. Definition.- 6. Number and Distribution.- 7. Physical Characteristics of Diffuse Nebulae.- 8. Dark. Nebulae.- 9. Cosmic Clouds.- 10. Distances and Dimensions of Diffuse Nebulae.- 11. Luminous Diffuse Nebulae.- 12. Proper Motions and Internal Motions (Visual).- 13. Radial Velocities of Diffuse Nebulae.- 14. Turbulence Effects in Diffuse Nebulae.- 15. Luminosity of the Diffuse Nebulae: Gaseous Spectra.- 16. Emission Variations.- 17. Luminosity of the Diffuse Nebulae: Reflection or Resonance Effects.- 18. Variable Diffuse Nebulae.- 19. Evolutionary Status of the Diffuse Nebulae.- c) The Planetary Nebulae.- 20. Definition of the Planetary Type.- 21. Number and Distribution of the Planetaries.- 22. Forms Assumed by the Planetary Nebulae.- 23. Proper Motions of the Planetary Nebulae.- 24. Distances and Dimensions of the Planetary Nebulae.- 25. Planetary Spectra; Spectrum of the Nebulous Matter.- 26. Planetary Spectra; Spectrum of Planetary Nuclei.- 27. Radial Velocities of Planetary Nebulae; Rotation.- 28. Spectroscopic Distribution Effects.- 29. Turbulence Effects in the Planetaries.- 30. Mechanical Theories of Planetary Structure.- 31. Quantum Theory and Planetary Structure.- 32. The Theories of Zanstra, Bowen, Carroll and Berman.- 33. Evolutionary Status of the Planetary Nebulae.- d) The Spirals.- 34. Historical Note on the Spirals.- 35. Apparent Distribution of the Spirals; Super-Galaxies.- 36. The Number of the Spirals.- 37. Conspectus of Forms Assumed.- 38. True Spirals.- 39. Barred Spirals.- 40. Elliptical Spirals; the Provenance of the „Minute“ Spirals.- 41. Irregular and Magellanic Type Spirals.- 42. Occulting Matter in the Spirals, and its Bearing on Observed Distribution.- 43. Proper Motions of the Spirals.- 44. The Spirals as a System of Reference.- 45. Internal Motions of the Spirals; Visual Determinations.- 46. Rotation of the Spirals; Spectrographic.- 47. Spectra of the Spirals; Stellar Type.- 48. Spectra of the Spirals; Emission Lines.- 49. Color Indices of the Spirals; the Results of Seares.- 50. The Radial Velocities of the Spirals.- 51. Distances of the
