  We provide the first demonstration that a neutron far-field interferometer
can be employed to measure the microstructure of a sample. The interferometer
is based on the moir\'e pattern of two phase modulating gratings which was
previously realized in hard x-ray and visible light experiments. The
autocorrelation length of this interferometer, and hence the microstructure
length scale that is probed, is proportional to the grating spacing and the
neutron wavelength, and can be varied over several orders of magnitude for one
pair of gratings. We compare our measurements of the change in visibility from
monodisperse samples with calculations which show reasonable agreement. The
potential advantages of a far-field neutron interferometer include high fringe
visibility in a polychromatic beam (over 30 %), no requirement for an absorbing
grating to resolve the interference fringes, and the ability to measure the
microstructure in the length scale range of 100 nm to 10 \mum by varying either
the grating spacing or neutron wavelength with a broad wavelength range and
single set of gratings.
