H. C. van de Hulst, Light Scattering by Small Particles (Wiley, New York, 1964), Secs. 6.13, 10.3, and 14.21.

M. Kerker, The Scattering of Light and Other Electromagnetic Radiation (Academic, New York, 1969), Sec. 3.9.2.

J. D. Jackson, Classical Electrodynamics, 2nd ed. (Wiley, New York, 1975), Sec. 9.14.

Ref. 1, Sec. 4.21, gives the classical derivation. Ref. 3, Sec. 9.14, gives a derivation that parallels more closely the quantum-mechanical optical theorem.

See, e.g., R. G. Newton, Scattering Theory of Waves and Particles (McGraw-Hill, New York, 1966), Sec. 4.2.2, especially Eq. (4.34), and the statement immediately following it. Note that Newton's *A*_{ii} and the *S*_{i}(0) of this Letter (and of Ref. 1) are related by *A*_{ii} = *ik*^{-1}*S*_{i}(0^{0}), so in relating Newton's result for *A*_{ii} to a statement about S(0) the terms real and imaginary and the terms even and odd must be interchanged.

See, e.g., Ref. 3, Sec. 7.10. The fact that Im ∊(*k*) and Re S(0) are both odd functions of *K* is no accident. If we construct a material that is a dilute suspension of *N* of our scatterers per unit volume, the dielectric constant ∊¯ of that material is related to *S*(0) by ∊¯(*k*) = 1 + (4π*Ni*/*k*^{2})*S*(0). That Im ∊¯ is odd in k implies that Re *S*(0) is odd and vice versa.

See, e.g., Ref. 3, Sec. 7.10.

Indeed, by using the dispersion relation for S(0) (e.g., Ref. 5), one can show by similar manipulations that [equation], where Im S¯(0, *k*′) = Im *S*(0, *k*′) - *k*′^{3} lim_{k′→0} [Im *S*(0, *k*′)/*k*′^{3}], exhibiting the λ^{-2} dependence of *C*_{tot} and identifying the coefficient in a different way. This and similar relations will be described in more detail elsewhere.^{9}

B. H. J. McKellar, M. A. Box, and C. Bohren, J. Opt. Soc. Am. (to be published).