Niels Bohr and John Wheeler set out at the same time to understand the theory of fission and they wrote the fundamental paper on the subject and came to the conclusion that it was the rare isotope of Uranium-235 which was responsible for fission. That, in a way, was perhaps good, because in order to make a bomb it would be necessary to... to separate the uranium isotopes which turned out to be a very difficult task. In fact the Germans who were of course very much aware of the possibilities, decided very early that the isotope separation was beyond their means. Not so the English. In England very soon two scientists, Rudolph Peierls and Otto Frisch came to the conclusion that a bomb could be made, and another refugee scientist, Francis Simon, concluded that it was possible to separate uranium isotopes by a fairly straightforward method which in the long run however took an enormous amount of... of technical and industrial development.