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 [Pg 204] 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 where is the Rydberg constant and an integer. For homologous components of successive terms of the same series, changes by unity, while the “residual” is sometimes practically constant (Rydberg’s formula), or, more often, is expressible in the form (Hicks’s formula), or (Ritz’s formula). In many cases this approximation fails for the smaller values of ; and prediction becomes very uncertain, though a plot of the residuals usually gives a smooth curve.... “The principles of selection, which determine what combinations among these numerous terms give rise to observable lines, are very simply expressed in terms of two sets of quantum numbers. “The azimuthal quantum number () is i for all terms of the s-series, 2 for those of the p-series, 3 for the d’s, 4 for the f’s, 5 for the g’s, 6 for the h’s, and so on. “Combinations usually occur only between terms of adjacent series for which the values of differ by a unit. A great many lines are, however, known for which the change of is 0, and a few for which it is 2. In the simpler spectra, such lines are faint, except when produced under the influence of a strong magnetic field; but in the more complex spectra they are often numerous and strong.