The distance between two plates of a capacitor is $\mathrm{d}$ and its capacitance is $\mathrm{C}_{1}$, when air is the medium between the plates. If a metal sheet of thickness $\frac{2 d}{3}$ and of the same area as plate is introduced between the plates, the capacitance of the capacitor becomes $\mathrm{C}_{2}$. The ratio $\frac{\mathrm{C}_{2}}{\mathrm{C}_{1}}$ is