John William Strutt, 3rd Baron Rayleigh

Waves of nearly the same frequency, though, can be added together so that they reinforce each other in one region and cancel in others, leaving compact bundles.

The speed of that bundle is called the group velocity, and Lord Rayleigh had shown how to calculate it in 1877.

When Duproy applied it to this scenario, the result matched the velocity of the particle exactly.

The bundle as a whole moves at exactly the speed of the particle.

The telegraph companies needed it for their cable contracts, and boxes went round the world.

Fifteen years later, Lord Rayleigh repeated the original experiment and found mistakes in it.

The ohm in the box was more than one percent too small.

And nobody could have told by looking at the box.

There is a diffraction of rays from their surface, and they look much bigger than they really are.

In 1899, Lord Rayleigh pointed out that a particle too small to be seen by the highest power of the microscope under ordinary conditions might be made visible if it received sufficiently intense illumination, and the ultramicroscope took advantage of this idea.

It occurred to Siedentopf and Sigmundi that if the particles in a solution could be strongly illuminated by a beam coming in, so to speak, sideways, then particles ordinarily invisible might stand out.

Their diffraction images, at any rate, would be seen.