The zero-field ac susceptibility \ensuremath{\chi}(\ensuremath{\omega})=\ensuremath{\chi}'(\ensuremath{\omega})+i\ensuremath{\chi}''(\ensuremath{\omega}) of the short-range Ising spin glass ${\mathrm{Fe}}_{0.5}$${\mathrm{Mn}}_{0.5}$${\mathrm{TiO}}_{3}$ has been investigated in the vicinity of the spin-glass temperature ${\mathit{T}}_{\mathit{g}}$ in the frequency interval 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3\char21{}}$5\ifmmode\times\else\texttimes\fi{}${10}^{1}$ Hz. Because of the influence of aging, \ensuremath{\chi}(\ensuremath{\omega}) is time dependent. After a temperature quench to a temperature T\ensuremath{\le}${\mathit{T}}_{\mathit{g}}$, \ensuremath{\chi}''(\ensuremath{\omega}) is found to decrease with time. The decay towards equilibrium is logarithmic in time and the corrections to the equilibrium values of \ensuremath{\chi}''(\ensuremath{\omega}) decrease slowly with increasing frequency. At temperatures Tg${\mathit{T}}_{\mathit{g}}$ in the vicinity of the freezing temperature of \ensuremath{\chi}(\ensuremath{\omega}), a weak increase of \ensuremath{\chi}''(\ensuremath{\omega}) with time is found. By use of linear response relations, the observed time dependence of \ensuremath{\chi}''(\ensuremath{\omega}) is shown to be consistent with results from time-dependent magnetization measurements. Furthermore, the existence of an overlap length scale in spin glasses has been verified by \ensuremath{\chi}''(\ensuremath{\omega}) and time-dependent magnetization experiments.