(a) Ultimate lines. (b) Temperature classes. (c) Furnace experiments. (d) Conductivity of flames. Solar intensities as a test of ionization theory. VIII. OBSERVATIONAL MATERIAL FOR THE TEST OF IONIZATION THEORY 116 Measurement of line intensity. Method of standardization. Summary of results. Consistency of results. IX. THE IONIZATION TEMPERATURE SCALE 133 Consistency of the preliminary scale. Effect of pressure. Levels of origin of ultimate and subordinate lines. Influence of relative abundance. Method of determining effective partial pressure. The corrected temperature scale. [Pg ix] X. THE IONIZATION TEMPERATURE SCALE 133 PART III ADDITIONAL DEDUCTIONS FROM IONIZATION THEORY XI. THE ASTROPHYSICAL EVALUATION OF PHYSICAL CONSTANTS 155 Spectroscopic constants (Plaskett). Critical potentials (Payne). Duration of atomic states (Milne). XII. SPECIAL PROBLEMS IN STELLAR ATMOSPHERES 161 Class stars. Class stars. The Balmer lines. Classification of stars. Silicon and Strontium stars. Peculiar Class stars. c-stars. XIII. THE RELATIVE ABUNDANCE OF THE ELEMENTS 177 Terrestrial data. Astrophysical data. Uniformity of composition of stellar atmospheres. Marginal appearance. Comparison of stellar and terrestrial estimates. XIV. THE MEANING OF STELLAR CLASSIFICATION 190 Principles of classification. Object of the Draper Classification. Method of classifying. Finer Subdivisions of the Draper Classes. Implications of the Draper system. Homogeneity of the classes. Spectral differences between giants and dwarfs. XV. ON THE FUTURE OF THE PROBLEM 199 APPENDICES I. INDEX TO DEFINITIONS 203 II. SERIES RELATIONS IN LINE SPECTRA 203 III. LIST OF STARS USED IN CHAPTER VIII 205 IV. INTENSITY CHANGES OF LINES WITH UNKNOWN SERIES RELATIONS 207 V. MATERIAL ON A STARS, QUOTED IN CHAPTER XII 208 SUBJECT INDEX 211 NAME INDEX 214 PART I THE PHYSICAL GROUNDWORK [Pg 3] CHAPTER I THE LABORATORY BASIS OF ASTROPHYSICS THE application of physics in the domain of astronomy constitutes a line of investigation that seems to possess almost unbounded possibilities. In the stars we examine matter in quantities and under conditions unattainable in the laboratory. The increase in scope is counterbalanced, however, by a serious limitation—the stars are not accessible to experiment, only to observation, and there is no very direct way to establish the validity of laws, deduced in the laboratory, when they are extrapolated to stellar conditions.