We present the results of an experiment proving in principle that negative (anomalous) dispersive transparent media can be realized. The Ca-transition 4s 2 1 S 0 ->4s4p 1 P 1 at λ=423 nm was chosen to investigate the absorption and index of refraction spectra of a two-level system strongly driven at resonance. For the simultaneous high precision measurement of loss and phase shift imposed on a probe field during the interaction with a chopped Ca-atomic beam we used a novel spectrometer which is based on a phase modulated Mach-Zehnder interferometer. So far, a dispersion of ω n 0 =-(3.1+/-1.0)x10 - 1 0 /MHz accompanied by residual absorption of α 0 =(0.125+/-0.040)%/cm has been achieved. These values are compared to the requirements which have to be achieved for a certain application of negative dispersion without absorption, that is for a broadband high build-up cavity (white-light cavity).