1. Richtmyer, F. K., Kennard, E. H., and Lauritsen, T., Introduction to Modern Physics (McGraw-Hill Book Co., Inc., New York, 1955), 5th ed., Appendix I, pp.633-635and Chap. II, p. 61.
2. Chandrasekhar, S., Radiative Transfer (Oxford University Press, Oxford, England, 1950), Chap. I, pp.1-2and Chap. VI, p. 147.