Nichols’ radiometer and discovering that radiation exerts pressure

As I write these words a comet speeds across our skies. Not that I’ve seen it – in London the stray light is so bright that such marvels are near-invisible. Johannes Kepler was the first to point out that the tail of a comet always points away from the sun, blown by the light; an idea that gained support from Isaac Newton’s corpuscular theory. In the 18th century, Leonhard Euler, a subscriber to Christiaan Huygens’ wave theory, proved that longitudinal waves too could transfer momentum to objects in their path, an idea developed by James Clerk Maxwell in 1873 when he unified electricity and magnetism. Shortly after this, Adolfo Bartoli suggested that radiation must have a pressure. It was an intriguing idea, but could it be measured?