In order to get white light from the dispersed light the prisms are arranged as shown in the diagram.
Recombination of the seven colors of the dispersed white light to get white light is known as Recomposition of white light.
Now remove the screen and place another prism P2 of the same material in the opposite direction. Place a white light screen behind P2. A spot of white light appears on the screen. Thus the second prism has recombined the dispersed light.