A tank contains three layers of immiscible liquids as shown in figure. The first layer is of water with refractive index $\raisebox{1ex}{$4$}\!\left/ \!\raisebox{-1ex}{$3$}\right.$ and thickness $8 cm$. The second layer is an oil with refractive index $\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$ and thickness $9 cm$ while the third layer is of glycerine with refractive index $2$ and thickness $4 cm$. The apparent depth (in $cm$) of the bottom of the container is _____ .