Linear polarization degree with polarization direction parallel to the solar limb, observed 25 arcsec inside the limb in the continuum and far from active regions, as a function of the wavelength. Observations: squares □ Leroy (1972, 1974), triangles △ Wiehr (1975), circles ° Mickey & Orrall (1974), crosses × Goutail (1978). Line: model result based on the HSRA atmosphere model (Dumont & Pecker 1971). From Leroy et al. (1977), used with permission. Red line: starting from 8000 Å, the polarization degree ceases to decrease following Rayleigh scattering on neutral hydrogen. It remains constant, as in Thomson scattering on electrons; HSRA modeling would give Thomson scattering dominant only after 10 000 Å.