[Pg 243] In the original paper will be found other tests of the Einstein equation, but the net result of all this work is to confirm in a very complete way the equation which Einstein first set up on the basis of his semi-corpuscular theory of radiant energy. And if this equation is of general validity it must certainly be regarded as one of the most fundamental and far-reaching of the equations of physics, and one which is destined to play in the future a scarcely less important rôle than Maxwell’s equations have played in the past, for it must govern the transformation of all short-wave-length electromagnetic energy into heat energy. [Pg 244] Fig. 34 V. HISTORY OF EINSTEIN’S EQUATION The whole of this chapter up to this point has been left practically as it was written for the first edition of this book in 1916. Now the altogether overwhelming proof that Einstein’s equation is an exact equation of very general validity is perhaps the most conspicuous achievement of experimental physics during the past decade. Its history is briefly as follows. As early as 1900 Planck[169] had been led from theoretical considerations to the conclusion that atoms radiated energy discontinuously in units which were equal to, or multiples of, , in which is the natural frequency of the radiator, and a universal constant which is now called Planck’s . He adopted the view that the seat of the discontinuity was in the radiator, not in the radiation after it had left the radiator, and in the second edition of his book modified the formulation of his theory so as to make this appear without any ambiguity. It was in 1905, as stated above, that Einstein definitely put the discontinuity into the radiation itself, assuming that light itself consisted of darts of localized energy, “light-quantas,” of amount . He further assumed that one of these light-quantas could transfer its energy undiminished to an electron, so that, in the photo-electric effect, the electron shot out from the metal with the energy , where represents the work necessary to get it out of the metal. In 1913 Bohr, in the development of his theory of spectra, without accepting Einstein’s view as to the seat of the discontinuity, assumed an equation which was precisely the inverse of Einstein’s, [Pg 245] i.e., he assumed that the energy lost when an electron jumps from one stationary state to another is wholly transformed into monochromatic radiation whose frequency is determined by equating the loss in energy to . In other words, Einstein and Bohr together have set up a reciprocal and reversible relation between electronic and radiant energy.