Cavity ring down measurements are performed on accumulation mode aerosol, 240 nm to 700 nm in diameter, over a range in wavelength, extending from 540 to 570 nm. We demonstrate that the measured variation in extinction efficiency with wavelength can be used to retrieve the dispersion in the real part of the refractive index. These measurements are contrasted with previous aerosol cavity ring down studies which have focussed on investigating the variation in optical extinction with particle size parameter through a variation in the sampled particle size distribution. In the measurements reported here, the gradient in the optical extinction can be recorded with fine resolution in size parameter (approximately 0.02) through variation in laser wavelength. Such an approach, as well as allowing a determination of the dispersion in refractive index, could be used to constrain the retrieval of refractive index at a single wavelength.