By explicitly dealing with the interaction of object and measuring instrument, von Neumann described a quantum mechanical measurement scheme consistent with wave function collapse. However, he did not prove the necessity of such a collapse. Although von Neumann's projection postulate is often presented as a normative description of quantum measurement, it was conceived by taking into account experimental evidence available during the 1930s (in particular Compton scattering was paradigmatic). Later work discussed so-called measurements of the second kind, that is to say measurements that will not give the same value when immediately repeated as opposed to the more easily discussed measurements of the first kind, which will.