I. INDEX TO DEFINITIONS
AN attempt has been made to define specifically, at some point in the text, most of the technical terms that are associated with the theory of ionization. For convenience of reference, the most important of these terms are collected into the brief index which is given below. The references are to the pages on which the term is defined.
| Atomic life | [21], [110] | Photosphere | [35], [47] | |
| Azimuthal quantum number | [8], [204] | Quantum number | [8], [204] | |
| Boundary temperature | [27] | Quantum relation | [11] | |
| Displacement Rule | [13] | Residual intensity | [51] | |
| Effective level | [135] | Reversing layer | [47], [49] | |
| Effective temperature | [27] | Rydberg constant | [14], [155] | |
| Excitation potential | [15] | Saturation | [52], [135] | |
| Fractional concentration | [105] | Series notation | [55], [203] | |
| Inner quantum number | [204] | Spectroscopic valency | [10] | |
| Ionization potential | [15] | Subordinate lines | [12], [100] | |
| Ionization temperature | [30], [132] | Temperature class | [24], [112] | |
| Marginal appearance | [105], [135], [179] | Total quantum number | [8], [205] | |
| Optical depth | [27], [35] | Ultimate lines | [11], [111] | |
| Partial electron pressure | [10] | Valency | [10] | |
| Partition function | [107] | Wings | [50], [179] |
II. SERIES RELATIONS IN LINE SPECTRA
A SYNOPSIS of the normal series relations in line spectra has been published by Russell and Saunders (Ap. J., 61, 39, 1925). A transcription of the passages containing definitions of spectroscopic quantities that are mentioned in the present volume is given below:
“Every spectral line is now believed to be emitted (or absorbed) in connection with the transition of an atom (or molecule) between two definite (quantized) states, of different energy-content—the frequency of the radiation being exactly proportional to the change of energy. The wave-number of the line may therefore be expressed as the difference of two spectroscopic terms which measure, in suitable units, the energies of the initial and final states. Combinations between these terms occur according to definite laws, which enable us to classify them into systems, each containing a number of series of terms, which are usually multiple—
“Any term
may be expressed in the form
