Bush said of his "As We May Think" memex device that "technical difficulties of all sorts have been ignored," but that, "also ignored are means as yet unknown which may come any day to accelerate technical progress as violently as did the advent of the thermionic tube." Michael Buckland concluded that Bush's 1945 vision for an information retrieval machine is unhistorically viewed in relation to the subsequent development of electronic computer technology. Buckland studied the historical background of information retrieval in and before 1939 because the Memex was based on Bush's work during 1938–1940 in building a photoelectric microfilm selector, an electronic retrieval technology invented by Emanuel Goldberg for Zeiss Ikon in the 1920s. According to Buckland, the legacy of Bush is twofold: a significant engineering achievement in building a rapid prototype microfilm selector, and "a speculative article" which through "the social prestige of its author, has had an immediate and lasting effect in stimulating others."