1. Earliest development of this topic in physics was made in connection with the theory of blackbody radiation. Compare Jeans, Dynamical Theory of Gases (Cambridge University Press, London, 1921), third edition, chapter 16
2. Fowler, Statistical Mechanics (Cambridge University Press, London, 1936), second edition, chapter 4.
3. Most of this section can be skipped at a first reading, but it should be scanned to see the main results concerning orthogonality of the wave functions (41 5) and the dynamical equation for a mode amplitude (41 10). The results are an application of the formalism used in quantum electrodynamics. Compare E. Fermi, Rev. Mod. Phys. 4, 87 (1932)
4. W. Heitler, Quantum Theory of Radiation (Oxford University Press, London, 1936), p. 40.
5. The literature dealing with special properties of resonators of particular shapes is becoming quite extensive. Some useful general references are: Stratton, Electromagnetic Theory (McGraw-Hill, New York, 1941), chapters 6 and 7