1. For interesting accounts of ancient and modern units see: Glazebrook, Proc. Phys. Soc. 43, 412 (1931); Gliozzi, Atti di Torino 67, 29 (1931).
2. 1. Planck, Theory of Heat Radiation,Blakiston, 1914, p. 173
3. 2. Lewis and Adams, Phys. Rev. 3, 92 (1914)
4. 3. Lewis, Phys. Rev. 18, 121 (1921)
5. 4. Lewis, Contributions from the Jefferson Physical Laboratory,Cambridge, 1922, Vol. 15