  The Sun's spectrum varies with center-to-limb distance. This variation is
governed by the underlying temperature-density structure of the solar
atmosphere. To explore the spectrally resolved center-to-limb variation (CLV)
we make use of two spectral atlases recorded with the Fourier transform
spectrometer (FTS) at the McMath-Pierce facility at Kitt Peak. One spectral
atlas obtained 10 arcsec inside the solar limb was recorded in 1978-79 as part
of the first survey of the Second Solar Spectrum, while the other atlas is the
well used reference NSO/Kitt Peak FTS atlas for the disk center. Both atlases
represent fully resolved spectra without any spectral stray light. We then
construct an atlas of the limb/disk-center ratio between the two spectra over
the wavelength range 4084-9950 \AA. This ratio spectrum, which expresses the
CLV amplitude relative to the continuum, is as richly structured as the
intensity spectrum itself, but the line profiles differ greatly in both shape
and amplitude. It is as if we are dealing with a new, unfamiliar spectrum of
the Sun, distinctly different from both the intensity spectrum (which we here
refer to with the acronym SS1) and the linear polarization of the Second Solar
Spectrum (for which we use acronym SS2). In analogy we refer to the new ratio
spectrum as SS3. While there is hardly any resemblance between SS3 and SS2, we
are able to identify a non-linear mapping that can translate SS1 to SS3 in the
case of weak to medium-strong spectral lines that are mainly formed in LTE
(being directly coupled to the local temperature-density structure). This
non-linear mapping is successfully modeled in terms of two free parameters that
are found to vary approximately linearly over the entire wavelength range
covered. These parameters and the various SS3 line profiles provide a novel,
rich set of observational constraints on model atmospheres.
