  In this note we use the notion of time-delay to explain the physical content
of the transformation properties of transmission and reflection amplitudes, as
a result of a displacement of the potential. Then, we reconsider the recent
analysis of the scattering problem by a finite-periodic potential, by Olsen and
Vignale, to obtain the total reflection condition in the limit of an infinite
number of cells. In doing this, we obtain an expression of Hartman's effect,
showing that the group velocity of the transmitted particle inside the
potential chain can become arbitrary large, as the number of cells tends to
infinity.
