  In 1918, H. Weyl proposed a unified theory of gravity and electromagnetism
based on a generalization of Riemannian geometry. In spite of its elegance and
beauty, a serious objection was raised by Einstein, who argued that Weyl's
theory was not suitable as a physical theory . According to Einstein, the
theory led to the prediction of a "second clock effect", which is not observed
by experiments. We briefly revisit this point and argue that a preliminary
discussion on the very notion of proper time is needed in order to consider
Einstein's critical point of view.
