The importance of the discovery can at once be judged by the fact that at one stroke the phenomenon not only confirmed Lorentz' theoretical conclusions with regard to the state of polarization of the light emitted by flames, but also demonstrated the negative nature of the oscillating particles, as well as the unexpectedly high ratio of their charge and mass (e/m). Thus, when in the following year the discovery of the existence of free electrons in the form of cathode rays was established by J.J. Thomson, the identity of electrons and the oscillating light particles could be established from the negative nature and the e/m ratio of the particles. The growing number of observations made by other investigators on studying the effects of using various substances as light emitters - not all of them explicable by Lorentz' original theory (the so-called «anomalous Zeeman effect» could only adequately be explained at a later date, with the advent of Bohr's atomic theory, quantum wave mechanics, and the concept of the electron spin) - was assembled by him in his book Researches in Magneto-Optics (London 1913, German translation in 1914). Not only has the Zeeman effect thrown much light on the mechanism of light radiation and on the nature of matter and electricity, but its immense importance lies in the fact that even to this day it offers the ultimate means for revealing the intimate structure of the atom and the nature and behaviour of its components. It still serves as the final test in any new theory of the atom.