  The "gravitational memory effect" due to an exact plane wave provides us with
an elementary description of the diffeomorphisms associated with soft
gravitons. It is explained how the presence of the latter may be detected by
observing the motion of freely falling particles or other forms of
gravitational wave detection. Numerical calculations confirm the relevance of
the first, second and third time integrals of the Riemann tensor pointed out
earlier. Solutions for various profiles are constructed. It is also shown how
to extend our treatment to Einstein-Maxwell plane waves and a midi-superspace
quantization is given.
