Abstract
Classical electromagnetic theory is used to study the constraints placed on interstellar graphite grains by the observations of Fitzpatrick & Massa (who found large variations in FWHM γ of the bump with minimal, and uncorrelated, variations in the central wavelength λ 0 ). Accurate calculations using the discrete dipole approximation are used to test the accuracy of the commonly used 1/3-2/3 approximation for graphite spheres. We show that the 1/3-2/3 approximation is sufficiently accurate for use in studying variations in the extinction profile due to changes in graphite grain size or coatings on the grains. We investigate the effect on the 2175 A extinction profile of (1) changes in size distribution, (2) changes in grain shape, and (3) coatings of ice or other material
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