  We investigate the infrared optical transitions between excitons of the
yellow, green and blue series in the cuprous oxide Cu$_{\mathrm{2}}$O. We show
that in many cases the dipole approximation is inadequate and, in particular,
that it breaks down in yellow-blue transitions even for moderate principal
quantum numbers of $n \approx 10$. The interband matrix elements of the
transition operator needed for the evaluation of the excitonic transition
strengths are derived from known as well as from fitted band parameters.
