When a white light passes through a hollow prism, then 

Inside the prism, which is hollow, the media is the same as that, outside the prism. The light rays on leaving the prism surface enter a lighter medium and bend away from the normal. Again, when the light enters a denser medium, the slab surface on the opposite side, light rays bend towards the normal. The light rays bend away from the normal when they emerge out of the denser medium of the second slab.
 ∴$\therefore$ The net result is that there is neither any effective dispersion nor any evident deviation.