  We show that by nullifying the short-wave response to the long-wave
excitation (local-field-effects), the adiabatic time-dependent
density-functional theory (TDDFT) of optics of semiconductors and insulators
can be brought into excellent agreement with experiment. This indicates that
the wing elements [($\Gv,\0v)$ and $(\0v,\Gv)$, $\Gv\ne\0v$] of both the
Kohn-Sham (KS) density-response function $\chi^s$ and the exchange-correlation
kernel $f^{xc}$ are greatly overestimated by the existing approximations to the
static DFT and TDDFT, respectively, to the extent that zero is a better
approximation for them than the corresponding values provided by current
theories. The head element of $f^{xc}$ is thereby fixed by the static
macroscopic dielectric constant $\epsilon_M$. Our method yields accurate
optical spectra including both the weakly and strongly bound excitons, while
its computational cost is extremely low, since only the head element of the KS
response matrix and the static dielectric constant are needed.
