For the description of an electron by a wave equation, the simplest equation available is the Dirac equation, which in the usual notation reads This corresponds to a point particle of charge e, spin ½ħ and magnetic moment eħ/2mc. This equation has met with marked success in describing an electron in static fields and slowly varying fields. For high-frequency fields, however, there are indications that the description is only an approximate one, and that allowance should be made for the finite extension of the electron.