An Analysis of the Shapes of Interstellar Extinction Curves. VIII. The
  Optical Extinction Structure
  New HST/STIS optical spectra were obtained for a sample of early type stars
with existing IUE UV spectra. These data were used to construct optical
extinction curves whose general properties are discussed elsewhere. In this
paper, we identify extinction features in the curves that are wider than
diffuse interstellar bands (DIBs) but narrower than the well known broad band
variability. This intermediate scale structure, or ISS, contains distinct
features whose peaks can contribute a few percent to 20% of the total
extinction. Most of the ISS variation can be captured by three principal
components. We model the ISS with three Drude profiles and show that their
strengths and widths vary from one sight line to another, but their central
positions are stable, near 4370, 4870 and 6300A. The Very Broad Structure, VBS,
in optical curves appears to be a minimum between the 4870 and 6300A absorption
peaks. We find relations among the fit parameters and provide a physical
interpretation of them in terms of a simplistic grain model. Finally, we note
that the strengths of the 4370 and 4870A features are correlated to the
strength of the 2175A UV bump, but that the 6300A feature is not, and that none
of the ISS features are related to R(V). However, we verify that the broad band
curvature of the continuous optical extinction is strongly related to R(V).
