  The nature of Galactic interstellar extinction is tested using reddening line
parameters for several fields in conjunction with equivalent widths
$W(\lambda4430)$ for the diffuse interstellar band at $4430$ \AA. The Cardelli
et al.$\;$relations [29] at infrared, optical, and ultraviolet wavelengths are
inconsistent with the newly-derived quadratic variation of $R_V({\rm
observed})$ on reddening slope $X$. A minimum of $R_V=2.82\pm0.06$ exists for
$X=0.83\pm0.10$, 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 \AA$\;$feature and a
correlation of normalized $\lambda4430$ 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 $R_V({\rm observed})$ 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.
