Roman craftsmen made a remarkable coup around 300 A.D. — they produced a cup that is red when lit from behind and green when lit from the front. The effect occurs because the glass contains tiny proportions of gold and silver nanoparticles that reflect light of certain wavelengths. The workers themselves may have discovered the technique by accident, and may not have understood it fully; only a few pieces of 4th-century Roman glass display this “dichroic” property. Art historian Donald Harden called it “the most spectacular glass of the period, fittingly decorated, which we know to have existed.” It now resides in the British Museum.