  Dimension reduction due to the orbital ordering has recently been proposed to
explain the exotic charge, magnetic and structural transitions in some
three-dimensional (3D) transitional metal oxides. We present optical
measurement on a spinel compound MgTi$_2$O$_4$ which undergoes a sharp
metal-insulator transition at 240 K, and show that the spectral change across
the transition can be well understood from the proposed picture of 1D Peierls
transition driven by the ordering of $d_{yz}$ and $d_{zx}$ orbitals. We further
elaborate that the orbital-driven instability picture applies also very well to
the optical data of another spinel CuIr$_2$S$_4$ reported earlier.
