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
