  We observe interference in the light scattered from trapped $^{40}$Ca$^+$ ion
crystals. By varying the intensity of the excitation laser, we study the
influence of elastic and inelastic scattering on the visibility of the fringe
pattern and discriminate its effect from that of the ion temperature and
wave-packet localization. In this way we determine the complex degree of
coherence and the mutual coherence of light fields produced by individual
atoms. We obtain interference fringes from crystals consisting of two, three
and four ions in a harmonic trap. Control of the trapping potential allows for
the adjustment of the interatomic distances and thus the formation of linear
arrays of atoms serving as a regular grating of microscopic scatterers.
