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The constraints placed in interstellar graphite grains by the observations of Fitzpatrick and Massa (1986) are studied by means of classical electromagnetic theory. 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. It is shown 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. The effect on the 2175-A extinction profile of changes in size distribution, changes in grain shape, and coatings of ice or other material is investigated. It is shown that all of the above effects produce correlated changes in both gamma and lambda(0). The calculated shifts in lambda(0) are such that the observed near-constancy of lambda(0) appears to limit variations in the size, shape, or coatings of small graphite grains.
Draine et al. (Wed,) studied this question.