  Measurements by the U.C. Berkeley Infrared Spatial Interferometer at 11.15
micron have yielded strong evidence for multiple dust shells and/or significant
asymmetric dust emission around NML Cyg. New observations reported also include
multiple 8-13 micron spectra taken from 1994-1995 and N band (10.2 micron)
photometry from 1980-1992. These and past measurements are analyzed and fitted
to a model of the dust distribution around NML Cyg. No spherically symmetric
single dust shell model is found consistent with both near- and mid-infrared
observations. However, a circularly symmetric maximum entropy reconstruction of
the 11 micron brightness distribution suggests a double shell model for the
dust distribution. Such a model, consisting of a geometrically thin shell of
intermediate optical depth ($\tau_{11 micron} \sim 1.9$) plus an outer shell
($\tau_{11 micron} \sim 0.33$), is consistent not only with the 11 micron
visibility data, but also with near-infrared speckle measurements, the
broadband spectrum, and the 9.7 micron silicate feature. The outer shell, or
large scale structure, is revealed only by long-baseline interferometry at 11
micron, being too cold ($\sim$ 400 K) to contribute in the near-infrared and
having no unambiguous spectral signature in the mid-infrared. The optical
constants of Ossenkopf, Henning, & Mathis (1992) proved superior to the Draine
& Lee (1984) constants in fitting the detailed shape of the silicate feature
and broadband spectrum for this object. Recent observations of H$_2$O maser
emission around NML Cyg by Richards, Yates, & Cohen (1996) are consistent with
the location of the two dust shells and provide further evidence for the
two-shell model.
