is equally difficult to see how there can be, in the atoms of a solid body, electrons having all kinds of natural frequencies so that some are always found to absorb and ultimately be ejected by impressed light of any particular frequency. However, then, we may interpret the phenomenon of the emission of electrons under the influence of ether waves, whether upon the basis of the Thomson-Einstein assumption of bundles of localized energy traveling through the ether, or upon the basis of a peculiar properly of the inside of an atom which enables it to absorb continuously incident energy and emit only explosively, the observed characteristics of the effect seem to furnish proof that the emission of energy by an atom is a discontinuous or explosive process. This was the fundamental assumption of Planck’s so-called quantum theory [Pg 254] of radiation. The Thomson-Einstein theory makes both the absorption and the emission sudden or discontinuous, while the loading theory first suggested by Planck makes the absorption continuous and only the emission explosive. The new facts in the field of radiation which have been discovered through the study of the properties of the electron seem, then, to require in any case a very fundamental revision or extension of classical theories of absorption and emission of radiant energy. The Thomson-Einstein theory throws the whole burden of accounting for the new facts upon the unknown nature of the ether, and makes radical assumptions about its structure. The loading theory leaves the ether alone and puts the burden of an explanation upon the unknown conditions and laws which exist inside the atom. In the first edition of this book, finished in 1917, I expressed the view that the chances were in favor of the ultimate triumph of the second alternative. In 1921, however, I presented at the Third Solvay Congress some new photo-electric experiments[185] which seemed at the time to point strongly the other way. These experiments consisted in showing with greater certainty than had been possible in earlier years[186] that the stopping potentials of different metals , , , when brought in succession before the same Faraday cylinder (see Fig. 35) and illuminated with a given frequency, were strictly identical. The significance of these results for the theory of quanta lay in the fact that I deduced [Pg 255] from them the conclusion that in the photo-electric effect, contrary to preceding views including my own, the energy “” is transferred without loss from the ether-waves to the free, i.e., the conduction electrons of the metal, and not merely to those bound in atoms. This seemed to take the absorbing mechanism out of the atom entirely, and to make the property of imparting the energy to an electron, whether free or bound, an intrinsic property of light itself. Fig. 35—Showing how photo-electric stopping potentials of different metals are compared by rotating and in vacuo into the position of .