Materials with certain values of the partial dispersion / 12 = [n(u1) mn(u2)]/[n(u1) mn(u3)] and the Abbe number x2= [n(u2) m1]/[n(u1) mn(u3)] are needed in different wavelength regions for the construction of dispersion corrected optical systems. Using a one-oscillator model forn(u) due to Wemple, we investigate the predictions of this model for the n - x -diagram and the / - x -diagram. The influence of a second oscillator is also discussed and compared with experimental results for optical glasses and crystals. The location of the second oscillator is correlated with that of the first one.