, 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, 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.
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
