Rik is the white light of the eye, Sâman the blue exceeding darkness. This Sâman (darkness) rests on the Rik (brightness). Therefore the Sâman is sung as resting on the Rik. Sâ is the white light of the eye, ama the blue exceeding darkness, and that makes Sâma.

In plain words: The passage compares the eye to the Sama chant. The bright white part of the eye stands for the Rik (a verse of the Rig Veda), and the deep dark part stands for the Saman (the sung chant). The dark rests on the bright, just as the chant is sung on top of the verse.