  In the fall of 1924, Enrico Fermi visited Paul Ehrenfest at Leyden on a
3-month fellowship from the International Education Board (IEB). Fermi was 23
years old. In his trip report to the IEB, Fermi says he learned a lot about
cryogenics and worked on two scientific papers, including the following one. It
was submitted in German to Zeitschrift fur Physik. The German version was known
to Weizsacker and Williams and cited in the papers (10 years) later in which
they extended Fermi's method to the Ultra-Relativistic case. The German version
was subsequently translated into a Russian version and perhaps other languages.
Fermi's Italian version (printed in Nuovo Cimento) is less widely known and
does not appear in the ``Collected Works''. Nevertheless, Persico remarks that
this was one of Fermi's favorite ideas and that he often used it in later life.
So, we would like to think of this as a late 100th birthday present to the
Italian Navigator. We would like to thank Professor T.D. Lee for his
encouragement of this project and for interesting discussions about Fermi. Also
Tom Rosenblum at the Rockefeller Archives for bringing Fermi's correspondence
to our attention and Bonnie Sherwood for typing the original manuscript.
