Predicted and observed optical spectra of the Crab Nebula. In concentric annuli, the mean and root mean square of the spectral indices s_O (I_ν ∝ ν^−s_O) have been determined from the data displayed in Fig. 1 of Véron-Cetty & Woltjer (1993). The superimposed curve is calculated from the intensity expected from the synchrotron model as described in Sect. 4.3 with the best-fitting parameters listed in Table 4. It is important to note that these data have not been used in the fit.