All things by immortal power,
That thou canst not stir a flower
Without troubling of a star.
Francis Thompson (1859-1907)
Picture a rainbow: under ideal conditions, you will not only see the primary and the (fainter) secondary rainbow, but you will also realize there is a zone between the two which is considerably darker than the surrounding sky, and you will see the series of very faint and narrow bands of colors, usually pink and green alternately, at the inner side of the primary bow. For centuries these narrow bands, called “supernumerary arcs,” have been a riddle to those who set out to explain the rainbow phenomenon scientifically.