The distance between two plates of a capacitor is $d$ and its capacitance is $C_1$, when air is the medium between the plates. If a metal sheet of thickness $\frac{2 d}{3}$ and of 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: