On June 3, 1880, Alexander Graham Bell transmitted a wireless telephone message from the roof of Washington’s Franklin School using light instead of a wire. His photophone directed speech toward a mirror. The voice made the mirror vibrate, and those movements altered a beam of reflected sunlight.

At the receiving end, the changing light was converted back into sound. The information did not travel as electricity through a cable; it rode on variations in brightness across open air. Bell considered the photophone his most important invention, even though it was far less practical than his telephone.

Weather supplied the obvious problem. Clouds, haze, or anything else interrupting the beam could interrupt the conversation. The photophone had demonstrated that light could carry a voice before technology could reliably shelter that light from the world between sender and receiver. Its strangeness lies in the literal mechanism: speech shook a mirror, the mirror edited sunlight, and a distant device listened to the flicker.