In general, the refractive index of light increases with the frequency of light (decreases with increasing wavelength, which is called "normal dispersion". There are, however, materials, where the refractive index decreases with the frequency of light ("abnormal or anomalous dispersion"). According to the Kramers-Kronig Relations, the frequency dependence of the refractive index is related to the frequency dependence of the light absorption in a material. Most optical media have the strongest absorption band in the ultra-violett range. This gives the normal increase of refractive index with frequency. When you measure the dispersion of the refractive index in a material above a strong absorption band, you can ( in a certain frequency range) get the "abnormal (anomalous) dispersion", i.e., a decrease of refractive index with increasing light frequency.