The nature of Galactic interstellar extinction is tested using reddening line parameters for several fields in conjunction with equivalent widths W ( \lambda 4430 ) for the diffuse interstellar band at 4430 Å .
The Cardelli et al . relations [ 29 ] at infrared , optical , and ultraviolet wavelengths are inconsistent with the newly-derived quadratic variation of R _ { V } ( { observed } ) on reddening slope X .
A minimum of R _ { V } = 2.82 \pm 0.06 exists for X = 0.83 \pm 0.10 , and is argued to represent true Galactic extinction described by A ( \lambda ) \propto \lambda ^ { -1.375 } .
It matches expectations for a new description of extinction in the infrared , optical , and ultraviolet by Zagury [ 32 ] .
Additional consequences , reddened stars with no 2175 Å feature and a correlation of normalized \lambda 4430 absorption with X , are not predicted by the Cardelli et al . relation [ 29 ] .
Known variations in X from 0.62 to 0.83 , and corresponding variations in R _ { V } ( { observed } ) from 4.0 to 2.8 , presumably result from forward-scattered starlight in the ultraviolet contaminating optical light of stars affected by dust extinction .
A new understanding of the true nature of interstellar extinction is important for establishing an accurate picture of the extragalactic distance scale , which in turn is related to our understanding of the nature of the Universe .