Parallel to this very complete establishment of the validity in the X-ray field of the Einstein photo-electric equation, and of its inverse the Bohr equation, has come the rapid working out in the domain of optics of the very large field of ionizing and radiating potentials which has also involved the utilization and verification of the same reciprocal relation. This will be seen at once from the definition of the ionizing potential of an atom as the electronic energy which must be thrown into it by bombardment to just remove from it one of its outer electrons. Through the return of such removed electrons there is in general a whole spectral series emitted. Similarly the radiating potential of an atom is defined as the bombarding energy which must be supplied to it to just lift one of its outer electrons from its normal orbit to the first virtual orbit outside that normal orbit. When this electron drops back there is in general the emission of a single-line spectrum. All this work took its origin in the fundamental experiments of Franck and Hertz[176] on mercury vapor in 1914. From 1916-22 the field was worked out in great detail, especially in America by Foote and Mohler, Wood, McLennan. Davis and Goucher, and others. Suffice it to say that whether the energy comes in the form of ether waves which through absorption in an atom lift an electron out of a normal orbit, so that the atom passes over to an excited or to an ionized state, or whether the energy enters in the form of a bombarding electron and reappears as a radiated frequency, the [Pg 248] reciprocal relation represented in the Einstein-Bohr equation has been found fulfilled in the most complete manner. In view of all these methods and experiments the general validity of the Einstein equation, first proved photo-electrically about ten years ago, is now universally conceded. VI. OBJECTIONS TO AN ETHER-STRING THEORY In spite of the credentials which have just been presented for Einstein’s equation, the essentially corpuscular theory out of which he got it has not yet met with general acceptance even by physicists of Bohr’s type. There seems to be no possibility, at present, of bringing it into harmony with a whole group of well-established facts of physics. The recent practically complete bridging of the gap between X-rays and light,[177] as well as that between heat waves and wireless waves,[178] with the perfectly continuous passage of the latter over into static electrical fields, appears to demand that, if we attempt to interpret high frequency electromagnetic waves—X-rays and light—in terms of undulatory “darts of light,” we also interpret wireless waves in the same way, and this in turn requires us to use a similar mechanism in the interpretation of static electrical fields. This brings us back to Thomson’s ether-string theory, which seems to be a necessary part of Einstein’s conception, if it is to have any physical basis whatever. [Pg 249]