D O C U M E N T 3 5 5 A U G U S T 1 9 2 6 3 5 1 355. To Gilbert N. Lewis [Berlin,] 22 August 1926 Dear Mr. Gilbert Lewis, Many thanks for your letter and your papers, which I read with much pleasure.[1] The considerations you offer in “The Nature of Light”[2] are the same ones that I have been agonizingly turning over in my own brain without ever finishing. The idea of an undivided effect on the four-dimensional distance 0 also looks very enticing but the sad thing is that if and is valid (more precisely, ), then it cannot be that . And yet the effect from A to C via a mirror B is just as intense as if A and C had the four- dimensional distance zero. Your mirror experiment (p. 28, fig. 4) really should be performed unfortunately, it is as good as certain—despite all this embarrassing in- comprehension—that all radiation pressures, averaged over time, are in conformity with Maxwell’s theory. I am fully convinced of your “law of entire equilibrium.”[3] But I cannot share your opinion that this law asserts that my derivation of Planck’s law is incorrect. This argumentation cannot be applied so simply here because a mechanism that catalytically favors spontaneous emission but not “positive and negative induced radiation” is inconceivable. Spontaneous emission and negative induced radiation, on the one hand, and positive induced radiation, on the other hand, must simply be regarded as inverse processes. I do not consider justified their separation according to the reasoning of the law of mass action, whereby negative induced radiation is to be regarded ¢somewhat² as a ¢third-party² reaction in which two quanta cooper- ate ( ( ). I just think that the law of mass action ceases to hold in its simplest form in the range of non-Wien radiation, i.e., in the range of degen- erate gases. While I was writing my theory, I myself perceived as unpleasant the non-evident reversibility of my statistical mechanism and also emphasized this in my presenta- tions on the subject. However, this is not a real objection. To see this, one only has to consider the radiative equilibrium of an oscillator in a Jeans radiation field, where one can operate with the classical theory and handle the considerations com- pletely analogously to those contained in my paper. One is thus certain of staying within the range of strictly reversible processes from the outset (mechanics and Maxwell’s equations). A B – 0 = B C – 0 = sAB 0 sBC 0 = = A C – sAC) ( 0= Zm hν + = Zn 2hν +