A new laboratory built in 1923 allowed Zeeman to continue to study the Zeeman effect. The laboratory was renamed the Zeeman Laboratory in 1940. For the remainder of his career he remained interested in research in Magneto-Optics. He also studied the propagation of light in moving media. This became important because of study into special relativity, and enjoyed keen interest from Lorentz and Einstein. Later in his career he became interested in mass spectrometry.