  Electron-hole pair excitations in the insulating cuprates $Sr_2CuO_2Cl_2$
were investigated by angle-resolved electron energy loss spectroscopy. The
optically allowed and optically forbidden transitions were observed to be
strongly anisotropic in $Cu-O_2$ plane. The former show a large energy
dispersion $\sim 1.5$eV along [110], and the latter appear at a higher energy
position ($\sim 4.5$eV) only along [100], but not along [110]. We interpret
these results as transitions involving excitons. A small exciton model is
examined to explain both the observed features.
