4 2 0 D O C . 2 7 8 T R A N S L A T I O N O F B O S E Published in Zeitschrift für Physik 26 (1924): 178–181. A manuscript is also available ([1 045]). [1]The abstract is not contained in the manuscript. [2]Planck 1901. [3]For a discussion of the inconsistencies in existing discussions of Planck’s law, see the Introduc- tion, pp. lxvi–lxviii. [4]See Einstein 1916j and 1916n (Vol. 6, Docs. 34 and 38). [5]In his letter to Bose of 2 July 1924, Einstein refutes the claim that his derivation made use of Bohr’s correspondence principle, and points out that Wien’s displacement law (which is used to fix the constants) is independent of wave-theoretical assumptions about the nature of radiation (see Doc. 279). [6]The assumption that light quanta carry momentum was implied by Einstein’s derivation in 1916 (see note 4). It had recently been confirmed by Compton’s experiments. [7]Bose here assigns polarization degrees of freedom to light quanta without wave-theoretical jus- tification (see Einstein’s comment in Doc. 279). According to later recollections reported by P. Ghose, Bose had in his original manuscript anticipated the concept of helicity by conjecturing that the factor of 2 arose from the photon’s spin: “On one occasion he [Bose] told me he was going to confide some- thing in me which I must never disclose. […] he got up and closed all the doors and windows so that nobody could hear what he was about to tell me. Then he started to explain to me in a low voice how in his derivation of Planck’s formula he got the first factor instead of as required, and that he had proposed in his paper that this factor of 2 could come from the photon having a spin of one unit which could be either parallel or antiparallel to its direction of propagation. But he said to me with a sad smile, the old man crossed it out! Einstein apparently replaced it by the statement that this factor came from the two states of polarization of light. There was no need to talk about the photon spin at that stage, was probably Einstein’s stand. And then he went on to remark with a dis- missive smile particle polarization what on earth can the polarization of a particle mean? I was shocked. I asked him why he had not pointed this out to Einstein when photon spin was eventually discovered. Einstein would have surely stood by your priority, I remonstrated with him. ‘How does it matter who discovered it? It’s been found, hasn’t it?’ was his reply with a sense of satisfaction” (Ghose 2009, pp. 407–408). [8]The second expression is missing a factor of V. [9]For a volume of 1 cm3 and a frequency of Hz (λ = 500nm) the number of phase space cells is . [10]Bose assumes for the derivation that the number of phase space cells per state s is a constant, is the corresponding Lagrange multiplier of a term . The assumption is irrelevant for the final result (see Cheng 2013, sec. 7.2.2). is the Lagrange multiplier for the condition that the energy be constant . [11]See Einstein 1924o (Doc. 283). 4πVν2 c3 ⁄ 8πVν2 c3 ⁄ νs 6 1016 ⋅ = As 3.3 103dνs ⋅≈ As λs λsAs λs¦pr s r = 1 β⁄ 1 β --E - 1 β¦Nshνs -- - s 1 β¦rpr -- - shνs s = =