ZEEMAN EFFECT, a phenomenon highly important in connection with theoretical physics. discovered in 1897 by Dr. Pieter Zeeman, of the University of Amsterdam. It consists in the doubling (or further multiplication) of the lines of the spectrum of a substance, when the source of the light under examination is placed in a powerful magnetic field. (See The results that are obtained under varying conditions are too complicated to admit of full discussion in the present place. In general. however, it may be said that when the ray of light under examination travels in a direction parallel to the lines of force in the field, each line of its spectrum is separated lines, which are collectively known as a •magnetic doublet.' The two components of such a magnetic doublet have (in general) the same intensity, and they are circularly polarised in opposite directions. Previous to Dr. Zee man's discovery, no source of light was known. the rays from which were completely polarised either circularly or otherwise. In order to ob serve the Zeeman effect properly, it is essential for the magnetic field to be both intense and uniform, and for the spectroscope to have a high resolving power. An idea of the order of magnitude of the phenomenon may be had from the following fact: In a magnetic field whose intensity is 10,000 C. G. S. units, each of the two well-known D lines of sodium is re solved into a doublet, and the separation of the constituents of either of these doublets is ap proximately equal to one-twelfth of the distance between the original I) lines.
When a ray of light originating in a mag netic field leaves that field in a direction per pendicular to the lines of magnetic force, the phenomena that are observed are much more consplicated. The lines of the spectrum are often observed to be triple, and not infrequently they arc quadrupled, or even more complex. In all cases, however, each constituent of the modi tit d line is comyletely polarized; but the polari zation, instead of being circular as before. is new plane. The several constituents into which each primitive line is resolved may, in fact, be divided into two groups, one of which is polar ized in a plane parallel to the lines of magnetic force, while the other is polarized in a plane perpendicular to those lines.
It will be observed that the Zeeman effect is due, apparently, to the influence of the mag netic field upon the ultimate molecular median ism which is concerned in the very genesis of the light-ray; And herein lies its great theoretical importance. The ordinary theories of light treat of the propagation of luminous waves through the ether and through matter, after those waves have left their source; and little is known of the way in which a fight-wave origi nates, at the atom which presumably gives it birth. The phenomenon discovered by 1)r. Zee man enables us to trace the consequences of modifying, by direct experimental means, the ultimate conditions that prevail at the '.cry source of the light; and for this reason it is likely to lead to results of the highest import ance with respect to the structure of matter and the nature of the ether and its motions. At the present day, physicists believe that the atoms •hich constitute matter are themselves com p•sed of far smaller particles called 'electrons' (q.v.), the electron, or ultimate unit in the structure of matter, being of the nature of a tiny disembodied charge of electricity. Light is believed to be due to the motions of these electrons within the atoms; and the Zeeman ef fect is believed to be due to the influence of the magnetic field upon the motion of the electrons. (Sc Ltratt ; ETHER; EticraoN Tit way ; SPEC Titnscory ). For a good account of the main facts, consult Cotton, 'Le de Zee man' (Paris 1900). Consult also Lewis. E. P., 'Fffeet of a Magnetic Field on Radiation' (Scientific Memoir Series,' Vol. VIII, New York 1903); Zet-maii. Pieter. 'Researches in Magneto optics' (London 1913). Reference should also he made to current scientific period icals. because the sithieet is being intently studied, and data in connection with it arc being rapidly accumulated.
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