Venerated as god and goddess, feared as demon and pestilence, trusted as battle omen, and used as a proving ground for optical theories, the rainbow's image is woven into the fabric of our past and present. From antiquity to the nineteenth century, the rainbow has played a vital role in both inspiring and testing new ideas about the physical world. Although scientists today understand the rainbow's underlying optics fairly well, its subtle variability in nature has yet to be fully explained.

Throughout history the rainbow has been seen primarily as a symbol--of peace, covenant, or...