The prism spectrum part of the experiment consists of a bright point source with condenser lens and iris, a 20 cm focal length cylindrical convex lens which focuses light onto a slit, and a 20 cm focal length spherical lens which focuses the light passing through a prism onto a distant screen. After the spectrum is aligned, the optical activity arrangement, consisting of a uniform plastic cylinder of Karo syrup between two rotatable polaroids, can be positioned between the cylindrical lens and the slit, as seen in the photograph. As either polaroid is rotated a dark band moves through the spectrum, because the polaroid absorbs the color/wavelength that has rotated by that angle.
The color of the light passing through the polaroid/Karo syrup system is a series of negative colors: white minus the color removed by the polaroids. The spectra are shown above (left to right): white light, cyan (minus red), magenta (minus green), and yellow (minus blue) respectively.