 It is interesting that Bohr’s initial rejection of the photon concept was also tied to the importance of being able to analyze radiation into its harmonic components, which was essential for the application of his correspondence principle (discussed below). As John Stachel notes, “It was indeed, his reliance on the correspondence principle that seems to have been a principal motive for Bohr’s distrust of the photon concept and his related willingness to give up energy-momentum conservation to save the classical electrodynamic picture of radiation” (Stachel 2009, p.72). Stachel argues that it was, in part, the results of the Bothe-Geiger and Compton-Simon experiments that finally led Bohr to embrace the concept of the photon. For these historical reasons, care needs to be exercised in invoking the photon concept when discussing Bohr’s early views; nonetheless the concept of photons can be very helpful in understanding the physics behind the correspondence principle (as well as Bohr’s views after the early-1920s), and hence, will be invoked in the following exposition.