The apparatus described by Jasperson and Schnatterly [Rev. Sci. Instrum.40, 761 (1969)] was modified to allow direct measurement of the derivatives of the optical dielectric functions of solids with respect to wavelength. The modifications are described and measurements of ∂&egr;1/∂&lgr; and ∂&egr;2/∂&lgr; are presented for metallic silver in the wavelength range 3300–4000 Å.