The analogy between the photon and the electron and the derivation of the quantum equation

On the view that the theory of the electron is exactly analogous to the theory of the photon, a quantum law is derived which is shown to require the existence of the first-order equation of the quantum theory. The notation required for this purpose is developed, and it is shown to have certain advantages. The limiting character of the fundamental lengths h/m0c and e2/m0c2 follows naturally, and the notation suggests that the latter should be regarded as a limit to the application of present physical theory.