It seems that Heisenberg in 1927 was thinking in terms of a classical diffractor. According to Bacciagaluppi & Crull (2009), Heisenberg in 1927 recognized that "the electron is deflected only in the discrete directions that depend on the global properties of the grating". Nevertheless, it seems that this did not lead him to think that the collective global properties of the grating should make it a diffractor with corresponding quantal properties, such as would supply the diffracted electron with a definite trajectory. It seems, rather, that he thought of the diffraction as necessarily a manifestation of wave character belonging to the electron. It seems that he felt this was necessary to explain interference when the electron was detected far from the diffractor. Thus it seems possible that in 1927, Heisenberg was not thinking in terms of Duane's hypothesis of quantal transfer of translative momentum. By 1930, however, Heisenberg thought enough of Duane's hypothesis to expound it in his textbook.