Pieter Zeeman
In 1896, shortly before moving from Leiden to Amsterdam, he measured the splitting of spectral lines by a strong magnetic field.  This research looked at what effect magnetic fields would have on a light source. He discovered that the spectral line is split into several parts in the by the magnetic field. Kamerlingh Onnes told Lorentz about Zeeman's observations on Saturday 31 October 1896 at the meeting of the Royal Netherlands Academy of Arts and Sciences in Amsterdam. The next Monday, Lorentz called Zeeman into his office and presented him with an explanation, based on Lorentz's theory of electromagnetic radiation. Zeeman's discovery is now  known as the Zeeman effect.