Up to 1914 no direct experimental proof had appeared for the correctness of this relation. In the photo-electric field discussion was active as to whether any definite maximum velocity of emission of electrons under the influence of monochromatic light existed, and although linear relations between energy and frequency had been reported by Ladenburg, Richardson and Compton, and Hughes, the range of frequencies available had been so small as to leave uncertainties in the minds of reviewers[170] and Planck’s had definitely as yet failed to appear. The unambiguous experimental proofs of the correctness of the foregoing theoretical relation began with the publication of the accompanying photo-electric results[171] which were reported briefly in 1914, and submitted in extenso in September, 1915. These were in a form to prove the correctness of the Einstein equation; for monochromatic light of known frequency fell upon a metal and the maximum energy of electronic ejection was found to be exactly determined by [Pg 246] as Einstein’s equation required. A year or two later Duane1[172] and his associates had found unambiguous proof of the inverse effect. A target had been bombarded by electrons of known and constant energy and the maximum frequency of the emitted ether waves (general radiation) was found to be precisely given by . D. L. Webster then proved that the characteristic X-ray frequencies of atoms begin to be excited at exactly the potential at which the energy of the stream of electrons which is bombarding the atoms has reached the value given by in which is now the frequency of an absorption edge.[173] This checks Bohr’s formulation of frequency-energy relations, since it shows that when an electron within an atom receives just enough energy by bombardment to be entirely removed from the atom, the total energy values of the frequencies emitted during its return are equal to the electronic energy of the original bombardment. De Broglie[174] and Ellis,[175] on the other hand, have measured with great accuracy, by means of the deviability in a magnetic field, the velocities of electrons ejected from different sorts of atoms by monochromatic X-rays, and have completely confirmed by such photo-electric work in the X-ray field my previous results obtained with ultra-violet light. They here verify in great detail and with much [Pg 247] elaboration the Einstein formulation where now represents the work necessary to lift the electron out of any particular level in the atom.