Light from arc lamp enters direct view prism emerges as spectrum.

Light from arc lamp enters direct-vision prism and emerges as spectrum. Collimated white light is sent through an adjustable slit and then through a direct-vision prism. The light is refracted, and the spectrum is focused on a projection screen. In this case, the spectrum is directly in line with the beam from the slit. That is, the direct-vision prism gives dispersion without deviation. The direct-vision prism used in this demo is made up of 3 crown glass prisms and 2 flint glass prisms. See figure 1. A flint glass prism produces about twice the dispersion as a crown glass prism. However, the deviation of each color by the flint prism is not twice that of the crown glass prism. The angles of both prisms are selected so that each prism alone will produce the same deviation for green light. After light has gone through the alternating crown and flint glass prisms, the green rays exit parallel to the original white light beam, and the other colors of the spectrum will be on either side. Ref: Modern College Physics by Harvey White, 6th ed., p.499-500