  In this paper we will be concerned with the explanation of the interference
and diffraction patterns observed as an outcome of the Young double slit
experiment. We will show that such explanation may be given {\it only} in terms
of a corpuscular theory, which has been our approach since the first paper of
this series. This explanation will be accomplished with an extension we make
here of the domain of applicability of the Born-Sommerfeld rules that we
derived in paper XI of this series.
