It is shown how it is possible, by means of geometry and the introduction of a principle of measurement, founded by analogy with the theory of H. Weyl, to discover a unity existing between gravitational, electromagnetic and quantum phenomena. Dirac’s equation and an extension of it are derived from the principle of measurement, and an essential feature of the theory is that it incorporates a theory of the electron in which its mass appears as a geometrical quantity, entering into the equation as a consequence of the existence of a fundamental unit of length. The observed mass is given by this geometrical mass together with interaction terms which also enter naturally into the theory.