The iconography of the Yale shield (or "arms") is taken from the University's seal, which was adopted by the Trustees of Yale College in the early eighteenth century. The unknown designer of the seal identified the book as the Bible by words which read "Urim and Thummin," probably names of sacred lots to be cast for the purpose of ascertaining the divine will.
When the Old Testament of the Bible was translated into Greek, the original meaning in Hebrew of "Urim and Thummin" was no longer known, and different words were used in different passages to translate them. Among the renderings given were "Light" and "Truth," and it is this interpretation which was chosen for the seal's legend and placed below the shield in Latin: Lux et Veritas.
The preceding text is adapted from The Arms of Yale University and its Colleges at New Haven, Yale University Press, 1963