1. SparnaayM.J., Physica 24, 751 (1958).
2. CasimirH.B.G., Proc. Kon. Ned. Akad. Wetenschap 51, 793 (1948). For a survey of later developments, focused around the notion of zero point energy, see T.H. Boyer, Annals of Physics 56, 474 (1970).
3. SchwingerJ., Particles, Sources and Fields, Addison-Wesley, Reading, Mass., 1970 and 1973.
4. It is also a response to the appeal of C.M. Hargreaves, Proc. Kon. Ned. Akad. Wetenschap B68, 231 (1965) ? ?it may yet be desirable that the whole general theory be reexamined and perhaps set up anew.?
5. See, for example, Equation (2?2. 48) of Ref. 3, with the Planck distribution introduced for the particle numbers.