The title and intro read: On the Theory of the Energy Distribution Law of the Normal Spectrum M. Planck Berlin (Received 1900) English translation from "The Old Quantum Theory," ed. by D. ter Haar, Pergamon Press, 1967 p. 82 GENTLEMEN: when some weeks ago I had the honour to draw your at- tention to a new formula which seemed to me to be suited to express the law of the distribution of radiation energy over the whole range of the normal spectrum , I mentioned already then that in my opinion the usefulness of this equation was not based only on the apparently close agreement of the few numbers, which I could then communicate, with the available ex- perimental data, but mainly on the simple structure of the formula and especially on the fact that it gave a very simple logarithmic expression for the dependence of the entropy of an irradiated monochromatic vibrating resonator on its vibrational energyà This formula seemed to promise in any case the possibility of a general interpretation much rather than other equa- tions which have been proposed, apart from Wien's formula which, however, was not confirmed by experiment.
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