  As London (1927, 376–377) remarks, one must admire Weyl’s  immense courage in developing his gauge invariant interpretation of  electromagnetism and holding on to it on the mere basis of purely  formal considerations. London observes that the principle of  equivalence of inertial and gravitational mass, which prompted  Einstein to provide a geometrical interpretation of gravity, was at  least a physical fact underlying gravitational theory. In contrast,  an analogous fact was not known in the theory of electricity;  consequently, it would seem that there was no compelling physical  reason to think that rigid rods and ideal clocks would be under the  universal influence of the electromagnetic field. To the contrary,  London says, experience strongly suggests that atomic clocks exhibit  sharp spectral lines that are unaffected by their history in the  presence of a magnetic field, contrary to Weyl’s non-integrability  assumption. London concludes, that in the face of such elementary  empirical facts it must have been an unusually clear metaphysical  conviction which prevented Weyl from abandoning his idea that nature  ought to make use of the beautiful geometrical possibilities that a  pure infinitesimal geometry offers. 