  We review the derivation of Planck's Radiation Formula on the light of recent
studies in its centenary. We discuss specially the issue of discreteness,
Planck's opinion on his discovery, and the critical analysis on the
contributions by Ehrenfest, Einstein, Lorentz, etc. We also address the views
of T.S. Kuhn, which conflict with the conventional interpretation that
discontinuty was already found by Planck.
