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 $\frac{2 \mathrm{d}}{3}$ 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