neutralize each other, so that there would be no horizontal motion at all. The vertical components, however, being in
FIG. 80 the same direction, will add to each other, so that the resultant of two beams of light polarized circularly in opposite directions and of equal intensity is a plane polarized ray.
To return, now, to Zeeman's phenomenon. Fig. 80 represents one of the sodium lines when examined in a direction at right angles to the magnetic field. The upper line represents the appearance when the light is polarized so that only horizontal vibrations reach the spectroscope. If, however, the polarizer is rotated through 90°, so that only vertical vibrations pass, the appearance is that of the lower half of the diagram, the two side lines appearing and the central line disappearing. Finally, if the light is examined in the direction of the magnetic field, which can be accomplished by boring a hole through the pole of the magnet, it is found that only two are visible—the two outside ones; and one of these is composed of light which vibrates circularly in the direction of the hands of a watch, and the other is circularly polarized in the opposite
FIG. 81 direction.
An extremely beautiful and simple explanation of this phenomenon has been given by Lorentz, Larmor, Fitzgerald, and a number of others. At the risk of introducing a few technicalities, I will venture to repeat this explanation in a simple form. For this purpose it is necessary to know that the particles or atoms of matter are each supposed to be associated with an electric charge, and that such a charged par-
