  The equivalent widths (W) of 565 spectral lines in the wavelength range of
4690--6870A were evaluated at 31 consecutive points from the solar disk center
(mu=cos(theta)=1) to near the limb (mu=0.25) by applying the synthetic
spectrum-fitting technique, in order to clarify the nature of their
center--limb variations, especially the observed slope differing from line to
line and its interpretation in terms of line properties. We found that the
distribution of the gradient beta (= -dlog W/dlog mu) well correlates with that
of dlog W/dlog T index, which means that the center-to-limb variation of W is
determined mainly by the T-sensitivity of individual lines because the
line-forming region shifts towards upper layers of lower T as we go toward the
limb. Further, the key to understanding the behavior of dlog W/dlog T
(depending on the temperature sensitivity of number population) is whether the
considered species is in minor population stage or major population stage, by
which the distribution of beta is explained in terms of differences in
excitation potential and line strengths. All the center--limb data of
equivalent widths (as well as line-of-sight turbulent velocity dispersions,
elemental abundances, and mean line-formation depths derived as by-products)
along with the solar spectra used for our analysis are made available as
on-line materials.
