  Weyl’s introduction of his Postulate made it possible for him to provide the first satisfactory treatment of the cosmological redshift. Consider a light source, say a star \(A\), which emits monochromatic light that travels along null geodesics \(L, L',\ldots\) to an observer \(O\). Let \(s\) be the proper time of the light source, and let \(\sigma\) be the proper time of the observer \(O\). Then to every point \(s\) on the world line of the light source \(A\) there corresponds a point on the world line of the observer \(O\), namely, \(\sigma = \sigma(s)\). 