Flux density ratio , where M = 1, is illustrated as a function of the Doppler velocity for the C IVλ1549 Å and Hα emission lines observed in the lensed quasar Q2237+0305 (red lines). μ(v) is shown after smoothing the line profiles with a 220 km s⁻¹ wide median filter and a 1500 km s⁻¹ wide median filter (thin and thick red lines, respectively). Outside the displayed velocity range, μ(v) becomes unreliable due to the low line fluxes. The continuum-subtracted spectra of images A and D ( and , respectively) are superimposed for comparison (blue and black lines, in arbitrary units).