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 \mum } \sim 1.9 ) plus an outer shell ( \tau _ { 11 \mum } \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 .