  We study the optical conductivity in doped nickelates, and find the dramatic
difference of the spectrum in the gap ($\omega$$\alt$4 eV) between one- (1D)
and two-dimensional (2D) nickelates. The difference is shown to be caused by
the dependence of hopping integral on dimensionality. The theoretical results
explain consistently the experimental data in 1D and
  2D nickelates, Y$_{2-x}$Ca$_x$BaNiO$_5$ and La$_{2-x}$Sr$_x$NiO$_4$,
respectively. The relation between the spectrum in the X-ray aborption
experiments and the optical conductivity in La$_{2-x}$Sr$_x$NiO$_4$ is
discussed.
