With regard to Zeeman's activities outside the field of the magnetic splitting of spectral lines, mention should first be made of his work on the Doppler effect in optics and in canal rays (laboratory tests). A second field of study was that on the propagation of light in moving media (justification of the existence of the Lorentz-term in the Fresnel drag coefficient). Other investigations were those into the influence of the magnetic moment of the nucleus on the hyperfine structure of spectral lines. He also succeeded, with J. de Gier, in discovering a number of new isotopes (38Ar, 64Ni, amongst others) by means of Thomson's parabola mass spectrograph. Zeeman's predilection for testing fundamental laws also found expression in his verification - carried out with an accuracy of < 1: 107 - of the equality of heavy and inert masses.