APPENDIX II 
401 
48. E. A. Milne. The Relation between the Spectral Energy Curve and the Law of 
Darkening of the Disc towards the Limb. Royal Soc. Phil. Trans. 223 A, p. 201 
(1922). 
49. R. Lundblad. The Radiation and Temperature of the External Photospheric 
Layers. Astrophys. Journ. 58, p. 113 (1923). 
50. B. Lindblad. Radiative Equilibrium and Solar Temperature. Nova Acta 
Upsaliensis, Ser. 4, 6 , No. 1 (1923). 
51. E. A. Milne. Absorption Coefficients and the Pressure of Radiation in the 
Photospheric Layers of a Star. Monthly Notices, 85, p. 768 (1925). 
( b) Chromosphere. 
52. E. A. Milne. An Astrophysical Determination of the Average Life of an 
Excited Calcium Atom. Monthly Notices, 84, p. 354 (1924). 
53. E. A. Milne. The Equilibrium of the Calcium Chromosphere. Monthly Notices, 
85, p. Ill (1924); 86, p. 8 (1925). 
(c) Reversing Layer. 
54. A. Pannekoek. Ionisation in Stellar Atmospheres. Bull. Ast. Inst. Netherlands, 
No. 19 (1922). 
55. R. T. Birge. The Quantum Theory of Band Spectra and its Application to the 
Determination of Temperature. Astrophys. Journ. 55, p. 273 (1922). 
56. R. H. Fowler and E. A. Milne. The Intensities of Absorption Lines in Stellar 
Spectra and the Temperatures and Pressures in the Reversing Layers of Stars. 
Monthly Notices, 83, p. 403 (1923). 
57. R. H. Fowler and E. A. Milne. The Maxima of Absorption Lines in Stellar 
Spectra. Monthly Notices, 84, p. 499 (1924). 
58. H. N. Russell and J. Q. Stewart. Pressures at the Sun’s Surface. Astrophys. 
Journ. 59, p. 197 (1924). 
59. P. A. M. Dirac. The Effect of Compton Scattering by Free Electrons in a Stellar 
Atmosphere. Monthly Notices, 85, p. 825 (1925). 
60. R. H. Fowler. Notes on the Theory of Absorption Lines in Stellar Spectra. 
Monthly Notices, 85, p. 970 (1925). 
61. C. H. Payne. Stellar Atmospheres. Harvard Observatory Monographs, No. 1 
(1925). 
The last-mentioned monograph contains in addition to Miss Payne’s own 
researches a full survey and discussion of existing knowledge of reversing layer 
problems. We have avoided entering on this part of the subject beyond the 
elementary principles. Any detailed study requires a knowledge of the series 
relations in optical spectra for which reference should be made to 
62. A. Fowler. Report on Series in Line Spectra. Physical Society, 1922. 
Nebulous Matter. 
Chapter xiii does not for the most part follow any published theory. The 
general ideas originated in private discussions with S. Rosseland to whom I 
am considerably indebted. Existence of interstellar matter was at that time 
hypothetical, but a month later observational evidence of a diffuse cloud in 
space was announced by 
63. J. S. Plaskett. The H and K Lines of Calcium in O-type Stars. Monthly 
Notices, 84, p. 80 (1923). 
Use has been made of 
64. A. Pannekoek. Further Remarks on the Dark Nebula in Taurus. Proc. Kon. 
Akad. Amsterdam, 23, p. 720 (1920).