  We consider various effects that are encountered in matter wave interference
experiments with massive nanoparticles. The text-book example of far-field
interference at a grating is compared with diffraction into the dark field
behind an opaque aperture, commonly designated as Poisson's spot or the spot of
Arago. Our estimates indicate that both phenomena may still be observed in a
mass range exceeding present-day experiments by at least two orders of
magnitude. They both require, however, the development of sufficiently cold,
intense and coherent cluster beams. While the observation of Poisson's spot
offers the advantage of non-dispersiveness and a simple distinction between
classical and quantum fringes in the absence of particle wall interactions, van
der Waals forces may severely limit the distinguishability between genuine
quantum wave diffraction and classically explicable spots already for
moderately polarizable objects and diffraction elements as thin as 100 nm.
